Why does my PC fan go to high RPM when not moving the mouse and low RPM when not moving mouse.

When doing normal things on the internet and stuff, and that's fine... then when watching a movie and not moving the mouse, the temperature goes up and the fan speed goes to about 32% and that's a little loud. and when moving mouse again the temperature goes down and the fan speed to a normal of 20%.

This is graphics card: 2GB AMD Radeon HD 7870.