Is this the first time they've wilted, or do they wilt and come back to normal healthy appearance? Almost all of mine wimp, as I call it, during the heat of the day and then rebound as it cools off. Same goes for me ;)

Some things that have caused wilt and sudden death here are boring stem menace near the soil level. Rotting tuber (dig up, cut off rot and replant which may save the plant.) Broken or damaged stem. Vascular disease- see link.

If it stays wilted, I'd start checking your main stalk/s for damage or soft/discolored spots, foliage for suspicious developments, then if still no obvious causes, I'd unpot it as carefully as you can and check the tubers. Also consider over-watering which is hard to do in a pot, but possible.

Good luck.

Here is a link that might be useful: some dahlia diseases causing wilt

hi poochella. thanks for the reply.

yes, they wilt in the heat and then they usually rebound once evening rolls around. the one branch that i had to cut off was not supported and probably broke when it wilted.

i realized the ones that wilt also are sitting about 3 feet away from a parked car. i had a thought after i posted that perhaps they were getting reflected heat from the parked car and that caused them to wilt. dahlias that weren't placed near the car have NOT wilted. i'm going to move those pots right now.

also, re: watering. i usually water once i know the soil is dry, so that rules out any possibilities of overwatering.