Spoilering this whole post for obvious reasons.
I am 100% certain that due to the general pacing and exposition issue the entire film had that there was probably some internal disagreements over drafts of scripts. Due to this, I believe that Ogrim was originally going to be a Blackrock Orc (like he was supposed to be) but someone, some producer or executive or someone, I don't know, complained that making Ogrim of another clan and of split loyalty might be "too confusing" for audiences. So they made Ogrim a Frostwolf (stupid change but whatever no big deal really), but Ogrim turns on Durotan during the little 'peace talk', later citing he 'couldn't bring himself to betray his own people'. I believe this was originally supposed to be Ogrim being unable to turn against his Chieftain and that originally, it may have even been Ogrim who ratted them out (but this much is speculation). Of course this is nothing but a firmly educated guess but I'm quite certain, based on what I can gather, that this was a plot rewrite that they sort of sloppily changed. One reason for the change being sloppy could be because they made the decision to make Ogrim a Frostwolf (and therefore have no reason to be loyal to Blackhand) AFTER they began some of the 3-D rendering for the scene, meaning the best they could do was write in a sloppy "but I couldn't betray our people!" line to try and explain it. Also I'm sure a lot of the Medivh stuff went through some sloppy rewrites because his stuff is easily the weakest in the film (the whole scene with the black square thing was just fucking terrible, right down to the pitifully fucking generic-as-hell 'darkness is light' thing)