Why is some of my bok choy collapsing into a slimy mess?

I planted bok choy for the first time this year, and overall it has been doing beautifully (2nd pic). However, over the last 1-2 weeks, certain leaves or even whole heads have been suddenly collapsing into a slimy mess, while others around them look totally fine (1st pic - this head looked fine 2 days ago). I‘m pretty sure I’m not overwatering. Any ideas what could be causing this?

u/wildyeast77 — 5 days ago