I.E. there is no particular reason (aerodynamically) to set the Mmo at 0.52 - except to mathematically make it "sufficiently below" VD/MD to prevent inadvertent exceeding of those speeds.
I find it hard to believe that an engineer would randomly select a Mach number for a certification criterion.
I'm thinking the 0.52 limit might have to do with vibration and buffeting, and/or stability. Increasing the AoA from level, 1G, flight will increase the speed of the air over the top of the wing, which is already going faster than the free stream speed. Depending on a few factors, you can get control force reversal as you increase the load factor - which usually results in an UNSAT for that exercise.