it's definitly not the decomp solenoid that's amking the starter keep turning over.
Here's an extremely simplified wiring diagram that my help.
you may have a faulty starter switch. A lot of older bikes have had these go bad, usually resulting in nonstarting.
bad starter relay or decomp control is likely, IMO, or a short somewhere there.
so you've shorted +12V to the Y/G going into the decomp control? that should make it keep trying to start all the time, assuming the start switch is out of the loop. if it's on the other side of the clutch lever, every time you pull the clutch lever, it should try and start, then. I can imagine that if the start swithc is in the loop, and you have a lousy switch (and return spring), a bump could occassionaly try to start the bike in the scenario you have described.