The bulb in the bottom light is a twin fillament type. The fillament for the tail light (5watts) has blown so when you put your foot on the brake the brake light fillament (21watt) glows....OR.... you have put a single fillament bulb in the bottom light (brake light hole) and this only illuminates when you brake.
See how this goes...compare the bottom bulb with the one in the other light cluster.
Cheers