Here are some photos. The first photo shows, the damaged pepper right next to a healthy cantalope. Pepper looked as healthy a few hours earlier.

Were the new plants fully hardened off before planting? Has it been hot and sunny?

If you believe that these plants are a total loss (no chance for recovery) I'd dig them out and examine their root system for any clues that may indicate the presence of soil pests, insects, grubs, mole/vole, etc. Seeing some leaves on the ground just after a few hours is odd unless an above ground critter like a bird was pecking at them.

I was thinking more that some more suseptible plants were affected by a "drift spray" of something like RoundUp. Your other plants may just take longer to die.

This is just way too bad.

Pasadena CA? Moles or gophers. They're all over the place. Very prevalent. Many removal services around. Dig around in the soil. Teeth are toxic and deadly to veges. "Hardening off" is next to irrelevant in coastal Southern California.

By asking about hardening off I hoped to find out if the plants had come out of a shaded sales area straight into blazing hot sun. Even in California I would imagine that is not a good idea.

Actually, in coastal California it never really gets that hot. Central California it is certainly a risk. Mean max in June in the LA basin is 90F, and mean high is 78F. In Texas, we laugh at summer temperatures like that. Do need to be a bit careful with intense sunlight, though.

The leaves look wilted and if there was drift spray, they would be more toasty looking. I suspect something going on at the root level, either furry creatures or a problem with how they were planted - maybe too deep or soil not tamped down around the roots. Our weather in SoCal has been very mild lately.

You said you "recently planted" those. Did you maybe transplant them too large (eg a pepper plant with a well-developed pepper seems a bit mature for transplanting)? And then was it hot sunny weather and you didn't water them each day? I live in the desert and after transplanting things out, I water them daily for several days until I think their roots have grown out into the surrounding soil and I can pull back on the frequency of watering.

Im betting on gophers. Thats what got me into container gardening in the first place. They look just like all my in-ground vegetables when I had gopher issues. I covered my backyard with bark and rock and no gophers and only grow fruit trees in-ground.

Thank you everyone. These veges were thriving for a week after they were planted. I think the culprit as some have stated is drift from weed killer. Recently I changed gardeners and after I posted, I asked him about weeds to be pulled out on a pathway near this area. He said he had sprayed the area a week earlier. I am a strict organic gardener and was shocked to hear this. It certainly has to do with drift. Just surprising that it takes a while for it to kill these plants. Another pepper which was thriving is gradually dying too. No wonder these chemicals have such terrible health consequences.

Pasadena is not coastal

"Pasadena is not coastal"

Duh? Pasadena CA in the LA basin. I go there regularly. About 10 miles from the surf of an ocean that covers a third of the planet.

In fact, Pasadena TX is right on the Gulf Coast, but since this question was cross posted to CA gardening, I suspect it's the former.

Gosh, these days, with the paltry traffic, it feels like 10 miles! Climactically, it is NOT "inland".

Pasadena can be more than 10 degrees warmer in summer and waay dryer than Santa Monica or Venice or Playa del Rey.

To people who don't live there it seems the same, but it is different.

Since you use Sunset climate zones, Pasadena is 21, Coastal is 24.

ZONE 21: Thermal belts in Southern California’s areas of occasional ocean influence

The combination of weather influences described for Zone 20 applies to Zone 21 as well. Your garden can be in ocean air or a high fog one day and in a mass of interior air (perhaps a drying Santa Ana wind from the desert) the next day. Because temperatures rarely drop very far below 30°F (–1°C), this is fine citrus growing country. At the same time, Zone 21 is also the mildest zone that gets sufficient winter chilling for most forms of lilacs and certain other chill-loving plants. Extreme lows—the kind you see once every 10 or 20 years—in Zone 21 average 28 to 25°F (–2 to –4°C).All-time record lows in the zone were 27 to 17°F (–3 to –8°C).

Coastal is 24

ZONE 24: Marine influence along the Southern California coast

Stretched along Southern California’s beaches, this climate zone is almost completely dominated by the ocean. Where the beach runs along high cliffs or palisades, Zone 24 extends only to that barrier. But where hills are low or nonexistent, it runs inland several miles.

This zone has a mild marine climate (milder than Northern California’s maritime Zone 17) because south of Point Conception, the Pacific is comparatively warm. The winters are mild, the summers cool, and the air seldom really dry. On many days in spring and early summer, the sun doesn’t break through the high overcast until afternoon. Tender perennials like geraniums and impatiens rarely go out of bloom here; spathiphyllums and pothos become outdoor plants; and tender palms are safe from killing frosts. In this climate, gardens that include such plants as ornamental figs, rubber trees, and scheffleras can become jungles.

Zone 24 is coldest at the mouths of canyons that channel cold air down from the mountains on clear winter nights. Several such canyons between Laguna Beach and San Clemente are visible on the map. Numerous others touch the coast between San Clemente and the Mexican border. Partly because of the unusually low temperatures created by this canyon action, there is a broad range of winter lows in Zone 24. Winter lows average from 42°F (5°C) in Santa Barbara to 48°F (9°C) in San Diego. Extreme cold averages from 35° to 28°F (2 to –2°C), with all-time lows in the coldest stations at about 20°F (–6°C).

The all-time high temperatures aren’t greatly significant in terms of plant growth. The average all-time high of weather stations in Zone 24 is 105°F (41°C). Record heat usually comes in early October, carried to the coast by Santa Ana winds. The wind’s power and dryness usually causes more problems than the heat itself—but you can ameliorate scorching with frequent sprinkling.

https://www.sunset.com/garden/climate-zones/sunset-climate-zone-los-angeles-area