Why do I get spin out and what fins will help reduce this?

The spin out, or cavitation, comes from some air that forms close to the profile of the fin.

This can happen at high speed, when you are landing a jump, or when you are following another sailor pretty close.

Also, spin out happens when you have too small of a fin compared the the board/sail size. If you are using a small fin on a big board and a big sail, putting pressure on your back foot will increase the spin out possibilities as you do not have enough surface to push on. This will create air close to the fin that will generate the cavitation and you will lose control.