A while ago, I walked up on one of our managers who was talking to a junior tech diagnosing an intermittent controls issue on a pool heat pump.

In the background, you could hear an EXTREMELY loud compressor.

The junior tech had just been moving some wires around, and the next thing he knew, there was a clack and then the noisy compressor and equalized pressures.

Do you know what happened yet?

I instructed him to shut it off and asked if it was a scroll compressor.

Sure enough, it was.

What happened was an instantaneous short cycle caused by the loose connector being moved. In that split second, the high-pressure gas in the scroll forced the scroll plate in the opposite direction ever so slightly. Once the power came right back on, it was running backward.
Now, this really shouldn't happen, but when it does happen, it's because of one of a few reasons.

Three-phase miswiring 

When a new three-phase building is constructed or a new unit or compressor is installed, it is possible to miswire the phases, resulting in a compressor running backward. This is not a good thing, but it can be corrected by switching any two legs of incoming power.

Instantaneous short cycling

This is what happened in the case of my junior tech. In most cases, the time delay in a board, thermostat, or controller will prevent this from occurring. Sometimes, the cause is internal to the system due to loose connections, etc.

Miswiring or Failed Capacitors

In single-phase applications, the run capacitor applies a phase-shifted potential that helps get the motor running and keeps it running. If the capacitor has failed or the compressor is miswired, it can occasionally (rarely) result in it running backward.

Failed Discharge Check Valve

Most scroll compressors have either an internal or external check valve that prevents the discharge gas from forcing back through the compressor, causing it to spin backward.

Occasionally, you may find a scroll compressor that makes a loud whirring once it cycles off. That can usually be corrected by installing a discharge check valve or by replacing the compressor if you choose.

Finally, be aware that anytime a scroll runs backward, it can do significant damage. If you find one that's running backward, shut it off immediately and correct the cause.

—Bryan