I've been trying to learn cleave for a little while now. I've killed 16-30 bears, about the same number of moose, and four lynx (and some number that I let get away cause I screwed up 1 shotting and wasn't on a horse). I generally have a Hirdsman's Sword equipped and get 20+ IP and keep my IP above 10. I also frequently sit there blocking until a new move opportunity comes up (or I've been letting the same animal poke at me for 10 minutes).
Is there a requirement I don't know about for learning Cleave? Am I just getting unlucky? Is it just fairly rare?