It's a synchronous motor that runs equally well in both directions, and the direction it goes is effectively random. (You could likely get it to always start in a particular direction by picking the point during the AC waveform to turn on the relay supplying power to the motor).
Did you ever wonder why, if you select 45 seconds of heating, the magnetron runs for 45 seconds but the motor continues to run for another
5 seconds after the magnetron is off? Because the turntable motor always runs for an integer multiple of the turntable rotation period, usually 10 seconds. So if you put something at the front of the turntable, near the door, it will be in that position again when the oven shuts off.Dave