Actually, it's a loss of tank control issue.
Similar problem:
https://forums.alliedmods.net/showthread.php?p=2133193
with exception: in my case, tank doesn't raise "player_incapacitated" event.
How to reproduce:
- spawn tank
- kick bots
- set fly mode to self and fly inside texture (e.g. under the tank)
- wait for ~ 30 sec. and return to tank = he is frozen.
P.S. Also some unfinished discussion:
https://forums.alliedmods.net/showthread.php?t=101998