Alright then, I was just browsing around Bulbapedia's list of glitches, when I learnt a glitch in SuMn apparently can make tag battle partner
Hau of all people
murder your 'mon with a Z-move in Aether Paradise's raid against them. I never had this since I chose Popplio myself, but those who chose Rowlet and thus have Hau took Popplio could have this happening. This is probably related to how the AI incompetently use Sparkling Aria due to its ability to heal burn of teammate while also damaging it.
It gets hilarious, and even more to see people's reaction. Some even took the time to take revenge on his Primarina. And people speculated on Hau's internal problems with this happening.