This has been observed a lot on the motorized bicycle forums I frequent.

On most (gas) motorized bikes, the engine-drive wheel is usually the rear wheel. Most of the time when someone gets a flat it is almost always the rear tire--which is especially charming since the rear tire is a much bigger bother to deal with than the front, due to the extra drive mechanism on the rear wheel.

The prevailing theory is that because the rear tire has torque applied to it, when you run over a sharp object laying oblong on the road surface the tire's slippage pulls the top of the sharp object backwards a bit, so that it stands up and then it pokes into the tire.