There are a lot of things that cause spin out, but what I have found on more than one occasion is that the foil shape or other subtle design differences between similar fins can have a huge impact on spin out.
For example, my iS 111 came with two Drake fins, a 38 and a 44, both with identical profiles and with rather thin/small foils. Both would spin out very easily. I also sanded them at Roger's suggestion with no improvement. So I bought a Tectonics Tomcat 42, that essentially had the same profile and rake as the Drakes, but a thicker/larger foil. The difference in spin out was HUGE and when I do push the Tectonics out, it will come back with very little effort. Getting the Drakes back in line after a spin out was significantly more difficult.
Formula 160; iSonic 111; HiFly Move 105; Tiga 263; '85 Mistral Superlight.
Maui Sails TR 11.0; 9.2; 8.4; 7.6; 6.6; Maui Sails Switch 6.0; 5.2; Maui Sails Global 4.5; 4.0.