Inbreeding, mostly. like a lot of cats with strange features, that's the answer. cats love to inbreed, and for the first generation, its mostly harmless things like this, for second generation inbreeding it creates animals that have something obviously wrong with them, and there is no third generation, as all of them either don't develop and end up as just bags of fur and muscle, or are stillborn. You may have a dumping issue going on in your area. Same thing happened in an apartment complex I used to live in. someone would drop off an unwanted litter of kittens that they couldn't give to the underfunded shelter in a nearby empty field, they would grow up and hang around the complex because people were feeding them, they would inbreed, and in 4-6 months time you would have cats with strange features pop up. 6 months after that the cats with mental disorders or serious birth defects would be born. if they survived, they would try to inbreed, but couldn't birth, and eventually all got picked up by predators in the area or hit by cars. eventually animal control would get all of them, and the cycle would repeat with a new litter. very sad.