The answers regarding built-up pressure are correct, but there's another aspect which hasn't yet been mentioned. In order for an induction motor to produce torque, it must have within it a magnetic field which is rotating at a particular speed (called the synchronous speed). Assume a particular motor is set up to run at a synchronous speed of 600rpm from 60Hz current. The magnetic field will then have six north poles and six south poles in a circle. When the "hot" wire is positive, the coils will try to drive the magnetic field so that the north poles are at the 12, 2, 4, 6, 8, and 10 o'clock positions, while the south poles are at 1, 3, 5, 7, 9, and 11 o'clock. When the "hot" wire is negative, the coils will try to drive the field so the poles are the opposite. If the motor is turning clockwise at slightly under 600rpm and a particular pole was at the 3 o'clock position at some point of time, then 1/120 second later that pole will be almost to the 4 o'clock position and the motor coils will try to pull it the rest of the way. If the motor was spinning counter-clockwise, then a pole which was at 3 o'clock at some point would be almost to the 2 o'clock position when the coils try to pull it the rest of the way. Note that the coils don't care which way the motor is turning--they rely upon its momentum for that.
To start such a motor, it's necessary to arrange things so that rather than simply going between two active positions, it goes between three or four. Typically this may be done by adding a capacitor and additional coils, so that on one line phase the motor will initially be pulled toward 12:00, 2:00, etc. but then soon thereafter to 12:10, 2:10, etc. Then on the next phase it will be pulled toward 1:00, 3:00, etc. followed by 1:10, 3:10, etc. Since 12:10 is a little closer to 1:00 than 11:00, the phase which tries to pull toward even numbers will apply a little clockwise torque. This amount of torque will be much smaller, however, than what could be produced if the motor were already spinning at a significant speed.
DC brush motors driven with a given voltage will produce maximum torque when they are starting or stalled. Likewise with AC induction motors which are driven with multiple "strong" phases. Most compressor motors powered by house current, however, produce near-zero torque at near-zero speeds. When there is no back pressure, the motors don't need to produce much torque to start moving; once they're moving, back pressure will increase, but so will their ability to produce torque. Shortly after a compressor is stopped, however, it will be unable to produce significant torque (since it's not turning) but will be unable to move without producing significant torque (because of the pre-existing back pressure).