It could be a number of things; nerves (like carpal tunnel), blood circulation being minimized, etc.
Stretching and taking frequent breaks can really help with both of the problems I've mentioned. I would also suggest that you are gripping the drumstick (or handle bars) too tight.
It could also be from the vibration of the drumstick when you hit the drum/cymbal, as well as the vibration of the handle bars on your motorcycle when you ride. I've even experienced numbness when mowing the lawn. For me, I was gripping the handle of the mower too tightly.
If your knuckles turn white, you are gripping too tight.
I've had some problems with this in the past, and it was attributed to Raynaud's Phenomenon which I was diagnosed with as a kid. Today, I don't have the problems because of exercise, stretching, proper diet, prayer and God healing me. I do struggle with the first two issues I mentioned, from time to time, because I don't take frequent breaks and stretch out like I should. Basically, I lose track of time and forget.
If the problem persists and you've dealt with things that you may be doing to cause it, be sure to check with your physician.
