If you are told your station was heard on 21.375 MHz but at the time you were operating on 7.125 MHz. What is one reason that could cause this to happen?

Harmonic signals are unwanted emissions that occur at exact integer multiples of your fundamental operating frequency. If you multiply your operating frequency of 7.125 MHz by 3, you get exactly 21.375 MHz. This proves your transmitter is radiating a third-harmonic signal. Defective power supply components, such as a filter capacitor or choke, would typically cause AC hum or a buzz on your transmitted signal, not place your signal on a completely different frequency band. Sending CW too fast might cause key clicks, but it will not multiply your operating frequency.