Board occasionally resetting when dropped or knocked
The resonator might be affected, but stopping the clock usually shouldn't cause a reset.

Are there any ceramic capacitors on the board? They can sometimes have sudden temporary changes in capacitance in response to shock/vibrations- this causes voltage spikes, which can easily cause a reset.

I'd watch the reset pin on a scope or logic analyzer and see if you actually see it trigger. If you do, it's likely the switch. If you don't, check the caps; if they're fine, I have no idea.