When in online mode and transitioning between zones. If you do not load immediately all your skeletons are lost. Not sure if this is intentional but I noticed they go away even before you properly load (aka if you eventually get a LE-65 matchmaking error).
Edit: This appears to only happen if you do not load in properly. As it seems to reset your minion status as you did not load in with them. If you load in properly with no error then minions appear with you.
Hope this helps!!