Why did HRE set up an anti pope when the pope is already a vassal?

In my most recent game, the Holy Roman Emperor has set up an anti-pope, but the thing is the current legitimate pope is a vassal of the HRE anyway? So why on Earth did he do that?

And aren't I, as follower of the OP (originial pope), suppose to get a CB on the HRE? I don't seem to have an option to declare war over his AP.

Here's the current pope. Note he doesn't like his Liege Kaiser Otto IV, but mainly because he controls the anti pope:

Here's the anti pope. Note he's also controlled by Otto.

After Otto died, it seems I then got my CB on his successor.