I was playing the cake event boss prestige mission guy, and I shield bashed through the party button right before the boss to try to get inside of the circular arena area why the guy was talking. I didn't actually get into the arena area and instead it kept me on the party button - I'd assume there is some sort of invisible wall around it while he's speaking. As soon as he stopped talking, I shield bashed. At first I thought that it was just my original shield bash finishing the animation, but the next time I stopped charging bombs I instantly shield bashed again. And again. And again. As long as I had max shield health and I wasn't charging an attack the game would instantly shield bash. I made it through without using a spark, but that was the most aggravating fight I've had in a while.
TL;DR- Shield bashed onto cake event boss party button so game thought that meant I wanted to shield bash every single time possible.