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It's not anything inherent about Mach 0.85; it's Mach 1 that's the problem. The reason airliners are usually somewhere between Mach 0.8 and Mach 0.9 is because of the critical Mach number of the airframe in question. When the relative airspeed of the entire aircraft is at the critical Mach number, there will be some part of the airframe where the airflow actually reaches Mach 1. This is due to air accelerating around the airframe as the airframe moves through the air. The critical Mach number will be different for different airframes, but it's usually the limiting factor for the cruising speed of airliners. This question explains more about the critical Mach number.