I don't understand how this kind of accident could occur! What happened?

RoundUp is translocated through the vascular system to the root system. If you used plain RoundUp and not one of the 'juiced up' products, damage can be lessened if the herbicide is washed off within a few minutes. Once RoundUp gets into the soil, it won't harm the plant (usually). However, if it is allowed to remain on the plant for a few hours, then the jig is up! Day temperatures, sunlight, etc. affect how quickly RoundUp will go to work. There are some formulations out that are very fast acting, and if you've used one of those then it is only a matter of minutes before the translocation process begins.

No, fertilizer won't help one bit.