My hair ties in knots by itself

When I lived in New York that happened to me constantly, even though my hair is in good condition and regularly trimmed. I now live in North Carolina and it only happens rarely. I don't know if it was the pollution, the water, the stress, or whatever else.The best way to prevent knots that I found was to usually wear my hair in some type of braided style. I also found that putting my hair in a pony tail usually made it worse.