I'd really investigate what might have upset him. Be careful, as we all know, the slightest thing could have set him off..... the sound of a new alarm clock, the existence of a new area rug, etc.

Allen will go outside the box on several occasions. 1) whenever we have a change in the family, like my son going back to college or returning home on breaks. 2) when he considers the litter level too low or not clean enough 3) when he feels "trapped" by the other cats.

I can't control #1 - my son has to come and go
I can control #2 but occasionally I lapse and then slap myself when he does it, because I know better
and #3: I have two boxes next to each other (uncovered) so that he can escape easily and use whichever box he happens to feel safer in at that moment.

Oh, and I swear by Cat Attract litter