That's pretty fast. I'm surprised they bothered to be honest - I'd expect a combination of a max IAS and a max altitude to override any need for a Mach limit. (i.e. any applicable Mach number ould be aoutside the IAS/Alt envelope).
It must be related to the gear mucking up the flow on the wing; maybe they had asym rolls due to that? You'd expect the gear - which is aerodynamically an ugly bit of junk sticking out of the wing - to be a great deal worse than the wing alone, which is why you might have a limit lower than Mmo say.
I'm still surprised a Mach limit was necessary, though.