If you've done (or heard about) the DK class campaign, you'll know that Bolvar's acting really weird. Really, really, weird.

To mages seeking Fel'melorn, Bolvar basically tells them that he doesn't really care what happens to them and isn't going to help them at all.

For Death Knights, he has them reforge the remnants of Frostmourne into a new weapon. Bolvar orders you to 'destroy' all the souls freed from Frostmourne's destruction. Not free, but destroy. One of the souls is that of Magroth, a paladin. Bolvar says, "Even when consumed by Frostmourne, the soul of Magroth refused to give up the Light. Show him the error of his ways."

Throughout the campaign, Bolvar seeks to make the Ebon Blade his tools/minions instead of merely allies. The only dissenter (and apparently free-willed) among the Ebon Blade, Mograine, is very conveniently killed at the end of the campaign, only to be raised again (by Bolvar) to become the 4th Horseman.

At the end of the campaign the player assumes the position of Deathlord as the new leader of the Ebon Blade, but in actuality the player is now a pawn of Bolvar along with the entirety of the Death Knights.

Wielders of the Blades of the Fallen Prince will sometimes view remnants of Arthas. One of these fragments tells you: "You believe that you are in control, that your will is your own... Yet you do as He commands. You exist by His whim alone. You imagine yourself to be free, but you will always be His instrument..."

This is obviously in reference to Bolvar.

Now for the conspiracy:

Why is Bolvar acting like a dick? Because he's being controlled by Kel'thuzad.

Kel'thuzad isn't dead and he's been quiet for a very long time. It definitely doesn't take this many years for him to reform, especially given how quickly he came back from being slain in the original Naxxramas. He's sneakily preparing to wage war against all his enemies simultaneously.

Look at the results of the DK campaign: Bolvar now controls the Ebon Blade completely because his only potential opposition was Mograine. Mograine is now Bolvar's minion, along with the 3 other Horsemen. Why does Bolvar send the Ebon Blade to attack Light's Hope, knowing that any attempt to raise Tirion will be unsuccessful? Because he's removing two obstacles simultaneously: Mograine and any paladins that happen to be around. Paladins are the only dangerous opposition to the Scourge.

Kel'thuzad loathes the Legion. During WC3 he pretended to be subservient in order to later betray them. Kel'thuzad will very likely view the Void under the same lense as the Legion.

Next expansion will probably be a 3-way war between the Scourge/Void/Legion.