Was he asleep when you checked on him? If he was, I can see that hair trigger snapping when you open the tub, then you pick him up to grab him and the stress just builds and lets loose because he thinks he's in extreme danger.

We have to remember, always, that they are not domestic animals, and to approach them with caution. It's always a good idea to wake them up before we handle them. Either by tapping on the cage, stroking their back with an inanimate object or the like.

If this isn't the case, and he continues to show aggression, you may have to consider some sort of injury or enviromental cue.

I personally would leave him be except for any needed reason to pick him up. Give him a week to unwind, and start from there.

I hope the best for you