Some folks choose to step away from the forum for a bit.
Some are shown the door because of their inability to behave. I don’t like commenting on either group out of respect for their privacy.
If you’re concerned, I would send a PM. Most members have email notification of PM (called conversations on Xenforo).
EDIT: There are some folks who are hit with a sudden illness, or worse, and they stop participating. We have no way of knowing which.