My usual response to threads like this is that when you are seeing a lot of one deck, you should obviously start playing a deck that counters it.
But I can't say that here, because there are NO matchups where Big Priest is at a disadvantage. So while I will point out that it's Big Priest, not Razakus Priest, that is dominating, I cannot prescribe a cure, because there really is a problem here.
The absolute worst win rate Big Priest has against any specific archetype is 50 percent. It's usually much higher. But 50 percent as your worst matchup? That puts you at Tier 0, and it cannot be allowed to stand.
When this happened with Druid after Frozen Throne came out, there was a nerf about two weeks later. Let's hope Team 5 acts as quickly this time around.