Outside A/C Unit Keeps Running 30+ Minutes After Inside Unit Stops
You've got a wiring problem, or a bad circuit board somewhere.

It's possible that the condenser unit is wired to run all of the time, but some protection circuit is kicking in and shutting it down. If that's the case, you would probably have ice build-up on your coils.

What is more likely is a relay somewhere is sticking, and remains closed when de-energized. If you provide the model number of you air handler and your condenser, I might be able to provide more direction.