Aeroelastic flutter. Same principal that causes a flag to flutter in the wind. Caused when the center of lift is ahead of the center of gravity of the control surface.

You can fix it (or reduce it) several ways.

Slow down.

Add a counter-weight to bring the C.G forward.

Tape the hinge line to seal out turbulance (sometimes helps).

Use beefier hinges, horns and connectors.

Make the aileron thicker than the trailing edge of the wing it attaches to.

Note that there are some planes - 3D models especially - that have so much control surface in relation to the wing that they might tear off if you fly too fast.