Why does my GE microwave fan turn on when I open the door?

Why does my GE microwave fan turn on when I open the door? Microwave was used for 12 minutes to bake a cake. When I took the cake out after the timer went off, the microwave fan kept running with the door open. The turntable didn't continue to turn. All buttons on the panel still work. The unit heats and the turntable turns when I use it. ANSWERS:
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