The kicking stuff is a bug.
I'm pretty sure it's intended, to avoid leechers.
If you die in the first wave, you'll be kicked when the second one (unless all your teammates also died).
If you die in the second wave, no problem. Third wave, same.
If you join, and it's the first wave, you'll be kicked. (dunno what happens if bots win though)
If you join, and it's the second wave, you'll be kicked. (dunno what happens if bots win though)
So to not get kicked after joining : stay alive the first wave, or die with all your teammates.