I don't think YM was so much a factor that "caused" the feud, but rather just just thrown in to fuel the fire. From what I've read, I actually think Yuzheng is more to blame for that since he's the one that actually started it by bringing out YM's name to slam Karen (saying the stuff about how Karen tried to convince him not to cast YM).
I'm pretty sure both sides' words should be taken with more than just a few grains of salt, but I personally found Yuzheng's attacks much more unprofessional (I mean, he decided to insult her personal life? wtf?). Perhaps that's why I think I distrust his words more between the two sides.