Because it has a fungal disease. Those dark spots on the dead blade are a dead giveaway. Where do you live? What is your watering schedule? How often and for how long?

That is weird that all over the net nobody recognizes fungal disease in St Aug. The first and last pictures show it crystal clear with the lesions running up and down the brown tipped blades. I have had rousing success fighting fungal disease with ordinary corn meal applied at 20 pounds per 1,000 square feet. Call your local feed stores to check availability. The cost should be well under $10. They might think you want corn gluten meal. You don't. The difference in price will tell you they are selling you the wrong product. CGM sells for roughly $30 per bag. When corn meal decomposes it attracts a fungus which attracts a predatory fungus causing the predator to over populate. Once the initial host fungus is gone, the predator goes after the disease fungus. Your alternative is to use chemicals. I can't help you with that. I've never needed them. But if you use chemicals first, then you cannot come back and try the corn meal later. The chemicals will kill off the predatory fungus from the soil. Try the corn meal first. If, after 3 weeks you don't like the results, either apply the corn meal again or try chemicals. Corn meal was the only thing that worked for me and was good enough to convince me to go full organic with my fertilizer. If you are happy with the corn meal after 3 weeks, you might just use it every month this summer to make sure you got it all. Corn meal is an organic fertilizer, so apply to the entire yard. Otherwise as the grass comes in over the dead areas, it would be much darker green. Watering: Deep and infrequent is the mantra for watering. This is for all turf grass all over the place. Deep means 1 inch all at one time. Put some cat food or tuna cans around the yard, and time how long it takes your sprinkler(s) to fill all the cans. Memorize that time. That will be the time you water from now on. My hose, sprinkler and water pressure takes 8 full hours to fill the cans. Your time will likely be less. I like gentle watering. As for watering frequency, that depends on the temperature. With temps in the 90s, deep water once per week. With temps in the 80s, deep water once every 2 weeks. With temps in the 70s, deep water once every 3 weeks. With temps below 70, deep water once a month. Note that you have to keep up with quickly changing temps in the spring and fall. This deep and infrequent schedule works in Phoenix and in Vermont, so it should work for you. The reason for deep and infrequent is to grow deeper, more drought resistant roots and to allow the soil to dry completely at the surface for several days before watering again. If it rains, reset your calendar to account for the rainfall.

Looks like it to me. Here is a link to some more information, which you might have already found. I would assume the grass came with the disease which got much worse with the rain. Take a note and don't buy grass from that supplier again. Being an organic kind of guy, I would be hitting the grass with ordinary corn meal at 15 pounds per 1,000 square feet every 3-4 weeks for the rest of the summer. This is INSTEAD of using a fungicide. As discussed in the link above, this is a soil borne disease. These problems are caused by an imbalance of beneficial versus pathogenic microbes - essentially an unhealthy soil. The fastest way to restore the soil to health is to use organic fertilizer, NOT fungicide. Corn meal has the additional benefit of heavily populating the soil with a predatory, but beneficial fungus called Trichoderma (try koh DER mah). Not all fungi cause disease. If you cannot find ordinary corn meal at your local feed stores (call first), then cracked corn will work. If you cannot find either one, then look for a grocery store catering to the Hispanic market. They will have 25-pound bags of corn flour. They will also have 25-pound bags of tortilla ready-mix which has baking soda and baking powder in it. The bags are almost identical, but you want the plain corn flour. Ask for help if you cannot tell them apart. If you have any doubts about the effectiveness of organic fertilizer, please ask. I think you ran into an unfortunate weather situation - unlucky timing. The grass could have been afflicted before you bought it, but that's hard to prove. It may have a guarantee or you might get a discount on buying again from the same supplier.

"isn't raining" is very rare here but it didn't rain Wednesday and only a sprinkle yesterday. Just applied another round of fungicide as I've now determined it's leaf spots. Can only hope it'll recover in 3 weeks. After looking at video of sod the day after it was delivered, I have little doubt this was a bad batch of sod. It was yellow and fell apart as we picked it up off the pallet. We never should have accepted delivery and I questioned it enough to make a video but waste of time going back to the sod company 2 weeks later; will just buy from another supplier next time.