It was a desperate plea for exposure.
Haystak had given Eminem props before on his songs, and had never said anything bad about him, even coming to his defense during the whole Source magazine feud. There was never any kind of indication that there was bad blood between the two up until Haystak started randomly (verbally) attacking Eminem.
It started at a concert. Haystak shouted at the crowd about how he was going to fuck Eminem up, or how Em didn't represent real whiteboys. Now, keep in mind that this was after the "I'm Having a Relapse" freestyle, but before Relapse was actually released.
Haystak's camp then put out a press release that was picked up by AOL Music and other online news outlets with the title "Eminem Gunned Down by Independent Rapper Haystak" or something close to that. He then started releasing diss tracks. After a few of those, he dropped a couple of videos.
"Something Someone Told Me", I believe were the titles (parts 1 and 2). He's making threats, and tries to allege that someone in the Shady Records camp had told him that Eminem only had like 4 songs, and that Relapse actually wasn't coming, which is laughable knowing now what we know about the Relapse sessions.
Then came the incredibly long-winded and pointless videos where Haystak "breaks down" the beef. He basically tries to sell the point that he's doing it because Eminem was being disrespectful of the Juggalos with his Relapse-era content, and that he was glorifying popping pills that kill people. Now, if you watch the videos, you can see that he's trying to look really angry, but it's obviously phoned in emotion. He also alleges he knew "some people" that brought guns to the video shoot (of We Made You) and that he had 10 G's to fly out and fuck Eminem up the moment he got a call about his location.
Eminem, of course, still didn't respond. Haystak got Brabo Gator to join in, and he dissed Proof. Haystak talked about a few lines where he was going to go at Hailie. Oktaine joined in after momentary hesitation because most of his fans were Haystak's followers. Haystak supported Oktaine at first (because Oktaine paid him several thousand dollars to do a collab album with him) but things eventually fell apart from there.
Haystak eventually gave up, because it got him nowhere. He's now back to making music about himself and his life, which is what he had been doing before he tried this ridiculous stunt. The reason he chose to diss Eminem at this point is because Em was at his weakest in many aspects. Not lyrically, but his comeback was bringing him newfound attention, and he wasn't sure of himself yet. That's also why DZK and Canibus chose to diss him around that time.
Haystak has a very dedicated fanbase, that worship him and question nothing he does. Seriously, it makes the Juggalos look like casual listeners. So he had nothing to lose, and everything to gain from his perspective. Unfortunately, he did lose credibility as an artist for those who actually use their heads, and I'm sure he got blackballed from any respectable venue or major album. It also came out that he was a registered sex offender.