Consider the environment they are working in.

The gas compressor is "squishing gas" right? If it were "squishing gas" from alow pressure (suction) into a higher pressure (discharge), then, if a motor were running providing the power to do that work, then the motor pulls current from the high voltage line, runs the motor, runs the compressor, and the compressor squishes the gas in the pipeline from low pressure at the suction to higher pressure at the discharge.

Fine. As long as everything runs properly.

Now, what if a failure occurs?

The high pressure side of the pump is - obviously, still at higher pressure than the low pressure (suction) side of the pump/compressor. IF this high pressure gas runs backwards (bleeds back) through the compressor blades and stator vanes, then it will turns the compressor rotor, which will turn the motor shaft, which turns the motor, which tries to become a generator and drive voltage back into the high volt AC lines coming into the motor.

The gas compressor (pump) becomes a motor driving a generator. Which fails promptly and often violently. Therefore, "motoring" is undesirable under almost all conditions. The term is used even if no "motor" is present.