Sometimes, when a team killer enters a vehicle, no other player can get in it after him.
To my knowledge, this bug was also present in—and probably inherited from—Arma 2, as I strictly remember having this same issue while playing Xeno's Domination 2. Thus, it is now as it was then, a common knowledge that a team killer always enters a vehicle last, after everyone else boarded.
The issue seems to be semi-random in nature.
I am guessing it might revolve around any player with a negative score and not just team killers. It's possible that it affects anyone who somehow got below 0 on the scoreboard.
Hard to pin point, but when it does happen then the game for some reason equates these type of friendlies to enemy combatants (player or AI), in case with whom you similarly can't enter an occupied vehicle of the opposing team.
I'm not sure this is a bug. Teamkiller switches to side enemy. If there were friendly AIs around he would have been dead already. Naturally you cannot ride with an enemy in your vehicle to stop abuse.
Technically, if it's a feature then auto-designating team killers as enemy does make sense as a simple solution for modifying the friend-or-foe stance to evoke a proper response from friendly AIs. However, in cooperative modes (PvE) you have to also account for possibility of unintentional TK, as well as a matter of any existing bugs (1), such as the one where drivers/pilots get unfairly punished for their gunners' mistakes (2).
1 - http://feedback.arma3.com/view.php?id=20339
2 - http://feedback.arma3.com/view.php?id=11593
I might be wrong though. Maybe it all plays out as intended and none of these issues are even related to each other.