As far as I know, many refrigerant compressors are not able to start up against a backpressure. Normally after stopping the compressor, after a while the pressures in condensor and evaporator will be equalised. So when the compressor has to start up again, the pressure in the dischargeline is equal to the pressure in the suctionline.
However when you connect multiple compressors in parallel to one evaporator and one condensor, the first compressor will be able to start up, but the second (and 3rd, 4th, etc) have to start up against a pressurised dischargeline.
This is needing much more torque and many units have not been built for that.
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