Situations like this are why we want guests to read the detailed rules here:
http://www.bfmracing.net/full_rules.php and also to ask questions just like this one. A lot of our rules can be misunderstood or have Grey areas that are difficult to explain in the server.
The rules clearly state that
you can exit a hog if it is stuck with a nade. There are two reasons this is allowed. One, it gives the occupants a better chance of survival. Two, if you are alone, you can get out to defend yourself, if you are still in danger. "Getting out to kill" is only against the rules as it pertains to someone who stops their racing progress for no good reason other than to rack up his kills. Defending yourself from imminent danger is always allowed. Defending yourself may even come in the form of
running over a walker that is trying to hog-jack you when you are in a lone hog. If you have a gunner though, you should let them handle killing of the walker while, as driver, you try to avoid the attack. Also if you are reasonably out of danger (past the hog-jacker) you should not circle back around, put yourself back in danger, just to get the kill (while claiming self defense).
Most of the time if you try to follow these guidelines in an honest way you won't get in trouble. On occasion though an admin may warn you because, in his opinion you took a little too much liberty with one of these allowances. Just take those warnings as an opportunity to learn what we feel are reasonable limits to what is "self-defense" and what is excessive slayer. No two admins will be exactly the same in this aspect but we strive to be reasonable about those situations and not look to give warnings every chance we get.
I hope this help a bit, feel free ask more questions to clarify if you need. I am sure you are not the only one with this question.