Not being a balancing expert I'd guess it was a combination of things such as an insufficiently rigid test stand and maybe the prop being used. The spikes are obviously harmonics and possibly the large spike might have been from the natural harmonic of the wood prop interacting with the natural harmonic of the test stand. But like I said, this is purely a guess.
One thing that intrigues me though is how you managed to get rpm figures that are so high. You've mentioned the small spike at 17K actual revs and no 61 will spin a 12x5 prop that fast. The graph goes to 20k.......
A 2 stroke engine will go into what's called 4 stroking mode if the mixture is so rich it can't fire every cycle but skips a cycle.
BTW, another area that causes vibration is that the prop doesn't turn at a fixed speed. It varies in revs through each cycle to the extent of a couple of hundred rpms's so the engine tends to rock sideways in both directions each cycle.