Like someone else said, it is unlikley that it has to do with the props, they would normally be limited in RPM to keep the tips of the blades from reaching supersonic speed, no real danger in that in itself, but certainly a loss of efficiency.
The limit probably has to do with the airframe structurally, ie windshield, wings or tail. IIRC, I have been told the Dash 8 has one Vmo speed below 14K due to the possibility of birdstrikes on the windshield and another Vmo above 14, but not sure on that one.
I know the Learjet I used to fly, an old 24 had a Vmo of 306, but on later models of the 24 it was 350, this was supposedly due to windshield testing.