Why would he even think of it?

Danny has a sippy cup of water and takes it into the bedroom to play with his brother. Ten minutes later I hear crying.  I go in to check.
ME: What happened?
Tommy: "I bit on Deeny."
ME: "You bit Danny?"
Tommy: "No. I bit on Deeny."
ME: "You sit on Danny?"
Tommy: exasperated. "No. I bit on Deeny."  He then mimmicks a hocking throat-clearing sound and (thankfully) pantomimes spitting on his brother.
ME: "You spit on Danny?!"
Tommy: "Alright."

Danny's water cup was empty, his hair and clothes were soaked. Tommy's shirt was soaked. Tommy got his butt smacked, all while crying "I dorry, Deeny. I yuh you. I dorry!"  Danny seemed relatively unscathed, nothing a boob couldn't fix.  But, why would he spit on his brother? What could make him think to do it, and then tell on himself for it, too?