Why are my vassal bishops paying taxes to the HRE Kaiser?

Several of my bishops are paying taxes to the Kaiser. Not only are they not paying the pope (although the pope is vassal to the Kaiser), their opinion of the Kaiser is less than that of me. One bishop has a 100 opinion of me and 32 for the Kaiser.

In the religion tab it shows a red hat and the opinion of the Kaiser. I thought maybe they're paying the Kaiser directly based on their opinion of the pope, which is 100 and the pope wins ties, but I don't know how to check their opinion of the pope since it's showing the Kaiser.

It sounds like the Kaiser has created an antipope. That means, that the bishops in the realm will pay taxes to the antipope instead of the old pope, and he will in turn give those taxes over to whoever appointed him. If the bishops don't like the new antipope, I assume they would continue paying taxes to their most direct liege (e.g. you).

There is a different question here on Arqade dealing with how to check the opinion between to NPCs. Perhaps that will help you figure out what's going on.

In the religion tab, hover over the button that creates an anti-pope. A popup will write something like:

He likes the Pope (100) more than us (100).

The numbers in brackets are the opinions you seek.

(Why the religion tab shows the opinion of the pope's liege is beyond me, as it is the opinion of the pope that matters in this case)