I think that the worship of dead people that permeates the religious world is some sort of idolatry. The reason I say this is that you can see in the Rambam that he defines idolatry as the worship of any being at all besides the First Cause. So worship pf dead people probably fits with that definition.

 The worship and love the religious have for the dead takes over their souls and bodies and they become zombies.=people with dead souls. 

That would be the reason for the signature of the Gra on the letter of excommunication. He apparently thought worship of dead people is not in accord with Torah.