I'm guessing the first box (the one you were standing on) wasn't pushed quite all the way? Even if it was, I think I know exactly what's happening: the box uses only one raycast to decide whether to start falling, and it must've gone over a small gap between the platform and the other box. I was just able to reproduce it myself. Should be a simple fix, but I'll have to wait until after the jam is over.
Anyway, thanks a lot for telling me about this!