Exactly same thing happened to me today. I only worked sometimes on the Bus driver, wanted to try a new job again and tried city route. Wasn't that good, so I choosed State route and when I arrived at El Quebrados, 4 zombies attacked my bus. I didn't really pay attention to them but yeah, I lost 4% of my Coach's health so why not add it, would be usefull for the civilians, bus drivers I mean.