Quite often it's because of some carbon build up on intake valves. After the bike has cooled off and sat cold for a while, the carbon becomes desaturated with fuel and will act like a sponge soaking up the atomized fuel mixture on start up. This causes the fuel mixture to go lean, which in turn causes the ECU to overcompensate. The lean-rich-lean-rich yoyo'ing is what cause the RPM hunting. After a while, the carbon deposits get saturated again, and the bike's fuel metering stabilizes again.
This happens most often in the spring time, because the engine has been run in cold winter months, where cold start fuel mixture can be very rich. That exacerbate the carbon build-up.
I usually just treat the fuel with Techron (or your favorite carbon deposit cleaner) and the problem goes away.