Things were tense in D12 before Proof died, and it seems like it's only gotten worse.
Denaun got a big head after he started making his own moves outside of D12, production-wise. Him and Von evidently had some sort of drama, and his relationship with Proof suffered from tension as well. Granted, I'm gathering this from some of Proof's lyrics, but I doubt the dude would just throw something like that out there for the world to hear if there wasn't truth to it. The fact that he kinda aired that laundry out at all shows that they must have passed the point of privately discussing it.
Proof and Fuzz didn't always get along either. Eminem mentions this in the Angry Blonde book. Could this be a source of contention for some of the other members? Even one of the other members? Possibly, but that's just a very weak bit of speculation.
Between that, and the feeling of being ignored by Eminem--along with Proof's death--had to have created problems in the group. It would have been great if they could have pulled it back together, but I'm not sure they can.
Bizarre doing it like this though? That's a dick move. No need to publicly leave the group, leaving everyone to only speculate what's going on. He didn't need to do that, and it doesn't look like it was talked out with the other members all that extensively.
Eye Kyu would be a nice replacement, being an original member and all, but it's not happening. I don't see another D12 album being released, unless it's a collection of unreleased tracks.