It does come and go by year. I think yellow jacket population has good and bad years, and you may also have a situation where there is a particular concentration along your route.

I've had them get down my shirt a few times. About half the time I can squish 'em before they get me. I just pull to the side and eject the body, and if I got whacked make sure I'm not still holding a stinger, and get on with it. The burn is usually gone within a couple of miles with the extra blood flow from riding anyway.