One of the theory is that James Holmes had intended for the apartment to be booby-trapped to create a diversion which means, if someone had opened the door to the apartment, it could very well create a multi-hysteria to lure most of the police/rescue workers to the apartment complex. While, having the shooting taken at the theater, it is possible that the plan was to have less resources there with more fatalities, and to get away with it.
So, I'm thinking, He probably figured that since he was 'caught' and his plan didn't go the way he wanted it to be - might as well go ahead and tell the police about his booby-trapped apartment.