German Shepherd/Brit Spaniel behavior problems?

My dog is a little older then 2 and a half years now and we've had him a few months longer then 2 years. Lately it seems like he's been becoming more protective/aggressive? We have a shock collar and we like to take it off when he's going in his cage (when we leave the house). This has never caused a problem before, but when we try to take the collar off while he's in the cage he gets very aggressive. Also, when we're about to close the door to his cage he growls and he's tried to bite a few times. What's causing this and how can we correct it? Thanks