I'd say it's genetic, I don't think anything can trigger it tbh. I had quite a physically abusive stepdad and learnt how to take a beating but I don't think it had anything to do with it. I have it and one of my brother's has it. Most people don't even know I have it, apart from my bestfriend and family. It's really frustrating having so much to add to conversation but not being able to, I've learnt to just talk when necessary and minimally.