So, I'm not really sure if I got it right, but I'm guessing Bucky is so cute that it got Steve flustered? --- Not exactly hehehe! Clint might have instructed Bucky in the gesture of the "fierce" sign and everyone thinks it's absolutely hilarious!
HEAR THIS: In the airport, when Black Panther is fighting Bucky, before Wanda helps him out, and T'Challa is about to sink his claws into him, you can hear Bucky yell "Do it! It's heartbreaking.<<<What I've just rewatched it and didn't notice.