That is called ‘Damping Off’ and it happens when the soil is too wet for the seedlings, or pots and soil are not sufficiently sanitized between uses.
Common mistake. We’ve all been there, done that! :)
This is kind of technical, but you’ll get the gist:
https://extension.umn.edu/solve-problem/how-prevent-seedling-damping
Hi, Diana - thanks. Since I was using a high end commercial “soil”, the Burpee’s Seed Starting Mix, and some of the containers were new (didn’t seem to matter -results were equally disastrous, new starter containers or old), I’m gonna run with the soil too wet. I don’t think I quite took well enough into account how astonishingly well these new “super soils” retain moisture and watered TOO “regularly” without rechecking the soil(s). Usually my problem is the opposite — I get overwhelmed with stuff to do, and don’t quite get back to the plants in time over a couple warm to hot low humidity days - so I get back to my starts a day too late to find they sprouted and then wilted in dry (soil) containers. :-(