Is it normal for my female bunny to 'hump' my other female bunny's head?

They are both one year old female rabbits, and one seems to like to hump/pester the other. Is this normal behavior? Or did I make a mistake in ing the rabbits? Every time Nilla does this to Choco, I check her to make sure she is female. By the way I see Nilla, she is female. Nilla always tries to 'hump' Choco's head or her back, and Choco gets really upset. Also, Nilla tries to sniff Choco's underside. Is this also normal? Thanks.