Why did Bud slap his hand over his mouth?

Bud slapped his hand over his mouth because he made a comment that he likely shouldn't have said outloud.

These guys really thought I was dumb. I said, "Yes, sir. But isn't it just like my luck to come clean across the state to find my daddy and he turns out to be a mean old coot?"
I slapped my hand over my mouth, I knew better than saying something like this out loud but it just fell out of my mouth before I could swallow it down.