The most common reason is getting reported by other players followed by investigation from a GM. Gathering ores and herbs used to be the most commonly reported activity. Personally, I think most of those reports were from other botters attacking their competition. In the last year it seems that BGBuddy (not to be confused with BGFarmer) has been the most widely reported bot. Don't use it! It is not safe. Everyone that has played WoW for more than a year knows what it is, and THEY WILL REPORT YOU! If you want to run BG's your best bet is to use Enyo and run them by hand. BGFarmer by Millz seems to have a lot of support as well. However, anything that may run you into a wall, or have half a dozen players all jump in unison 12 steps out of the gate, or cause bot like behavior in a BG is possibly going to get you reported.