We all know how Bakugo wants to be the strongest so when he learned that frowning uses more muscles he would automatically frown as to improve his facial muscles.

Of course he would of rubbed it in Midoriya's face because they are both so childish. It doesn't really do anything in terms of making him stronger but he seems to think so.

Sorry for updating late again, it's just really easy to fall asleep to a cat purring.