Adrenaline, most likely. You're more focused, most likely.
I don't think it's a problem, if that's what you're wondering. I pitched angry constantly -- my teammates avoided me on game day. I mother-fuc*ed opposing hitters under my breath (and often audibly) every single pitch. I was a psycho It worked for me, though.
I was a closing pitcher in the Chicago Cubs organization, so this mentality worked well. Was one of the reasons I got the role. But even as a starting pitcher in college, I approached each inning like a closer approaches the 9th inning of a game -- I pitched pissed off at the world with a chip on my shoulder. Loved it.