People don't do it by mistake. It is deliberate trolling. We need a term for it like "vehicle jacking" and then have the bastards kicked/banned. I believe that sgt. shanko is notorious.
Don't underestimate the stupidity of people. I have encountered numerous occasions of people jumping in your Pak or AA vehicle and start moving around. In most cases these were relatively new players, who just wanted to help or have some fun themselves and they just don't understand the situation. They may even think you are blocking an AA vehicle or AT gun, because you are "behind" the frontline and rarely have targets. Therefore they conclude your gun is wasted, while instead you are waiting in an ambush position and keep the gun alive.
In a very few occasions I have encountered people jumping around 'your' pak, with the sole purpose of annoying you (and possibly getting in the PaK themselves).
BTW: This brings me to another stressor: People wasting assets for there own stupid suicide missions. Just driving tanks, AT guns or whatever to the frontline and doing one or two kills. Afterwards your gun is gone and all your tools to defend a certain flag. Yet these people actually think they have done something usefull.