Now that you mention it I remember this as well.
From what I remember, and I could be wrong, i think he took a phone call or something and talked with someone 50 was beefing with, or did a show with someone he had beef with, and em was mad so he stopped talking and working with him. I'm almost positive it wasn't something that directly affected or dealt with em, and I'm pretty sure it dealt with 50 and lantern talking to someone he was beefing with.
hopefully someone knows exactly what the problem was

Edit: I was real close, lantern left because of a phonecall with jadakiss that was taped, and since 50 beefed with jada, when the convo was released, instead of causing drama (and the fact lantern believed em and 50 wouldn't like it) he left shady records completely. Here's an article about it:
http://www.mtv.com/news/articles/150140 ... tern.jhtml