Question: how did hodor become hodor?
Answer: In the future, Hodor is being told to "hold the door" to give Bran a chance to escape the undead army. Young Hodor, connected to future Hodor through Bran's presence in the past, starts having some sort of fit and hears Meera Reed yelling at him to "hold the door." ... Hold the door" becomes "Hodor." Poor Hodor.