what happend?

ok, from what I've read roots turning black is usually caused by nute burn, or over watering, can roots "rot" by lack of watering, this happened to 2 of my seedlings:


they basically fell out of the soil, I wasn't too hopeful, but I pushed them into a wick, one of them seems to be recovering, (the root is continuing to grow but the other on (the one pictured) didn't, if anyone can tell me there guess as to what happened, I would like to know in order to prevent it from happening next time.

i hate bumping myown threads, but i would really be interested in what happened, it was grown in clean no-name brand soil, good drainage container, no nutes, water might have been a bit high on the ph scale though