It is because people don't like what they do not know. They also have a habit due to a chemical inbalance in the brain to always be right, or always want to be right, alot of people just keep their traps shut normally, but you get the odd one or two who don't.
Basically, a muslim will hate another muslim because they think that the other is not being a good enough muslim, for certian reasons, mostly due to person A not doing things how person B does them, therefore critisism is due, in other logic.