We’re not aware of anything in particular that drives it, although generally we schedule them to close at times when there are several people actively engaged (as they know the topic is going to be closed and try to sneak in a few final words right before the deadline).
Not sure why that would have any effect, especially given I don’t think our site is big enough to ever have that be more than like 20 people (at absolute most, 50, but that would have to be an EXTREME edge case).
We also generally get up to a few thousand posts before we end up scheduling things to lock, so maybe longer topics handle it worse than topics with like 100 posts?