Snape was mortified at having his boxers waved about while he was still in them.
I think the most mortifying part was that his secret love -Lilly- was there, and worse, she tried to protect him. He lashed out at her to cover his own shame and embarrassment, a very human (and especially teen-aged human!) reaction.
I think I read that part in the British book I have. I’ll have to dig it out.