It's gotta be instinct.

So Brad thinks the babies have an instinct to do this:

Because who in the world has pouted to this extent to teach him how?!

He struck this pose long enough for me to get my camera and take several pictures. 

He was mad at Blair because she hit him. He was doing something she didn't want him to do - playing in Gus' water, I think. I think she hurts his feelings because he just wants to be like her. He copies everything she does. (And she is not supposed to hit him, of course. You know how it goes....) But yes, he hits back. 

He got some looks that's for sure. He got it honest.