It's probably just that you're usually turning one way more than the other. Deflate your tires, take them off the rims, flip 'em, inflate them, and install them. Then the wear will begin on the other side too.
If it's on the left side then it is probably due to the road. I guess flipping the tire should be ok but tires are meant to be used in one direction. This is indicated by the direction of the treads. I see nothing wrong with this so flipping the tire would be the way I would go.
Oh whoops, I was thinking road bike... I usually see colors on road bikes but they're on mountain bikes too... Ya don't flip the tires. It's not a big deal anyways. You'll have more traction on the roads now anyways.