Could be a short in "across the start switch button wire connections" or it could be the start relay contact "sticking", or it could be a "broken spring" in the solenoid that allows the contacts to come together engaging the starter when you hit a bump.
The first thing is, did you do some recent electrical work around the handlebar controls, starter drive or near the fuse/start relay areas? I would start where you did any rework that required you to "dig in" and inadvertently caused some collateral damage.