It could also be a molt. Coturnix molt twice a year; once in the fall after the breeding season ends and once in the spring when breeding season begins. Almost all my birds are in molt right now. The season is just about wrapped up for me.
The key difference between plucking and molting is that you will see a whole bunch of pinfeathers coming in. At the start of the molt, you will notice your bird's feathers come out real easily. You might also notice a change in personality and they might not enjoy being handled. They can become quite grumpy. Some birds molt more aggressively than others and it's also not unusual for them to lose some weight. Molting is also a period of immunosuppression for them so watch out for diseases.