It could possibly be a glitch in the mod, if you take a certain vehicle, say the Hunter from the army base, it may have desynched and respawned on someone elses screen, and then they innocently go to enter it, and as soon as they get in it the game has a fit and throws you out, and it appears that you've been hijacked.
The speedhacking though sounds definately like a cheat, it's very easy to cheat in MTA:VC and a lot of people take advantage of that.