I have a collection of 3 week old seedlings. They are outside.

I've had 2 in as many days just fall over. Upon investigation, saw stems had just withered away to a little tendril at the soil. The little withered "strand" was really stringy (tough) and not brittle or dried out. Below it was regular stem and roots.

I repotted both so I could bury them deeply (pretty much up to their leaves) Didn't expect them to make it but they're both trucking on, despite the 100+ oven temps here.

I really don't like them just falling over! I'm very sure it wasn't errant wandering animals or hail/rain damage ect. Any thoughts?