What you were expecting to happen, and what actually happened:
I was expecting to see battlecrashers in PVP & Explore. Multiple runs & PVP battles have given 0 battlecrashers. How often does this happen? When did it begin happening?
All the time. May be related to the disappearance of Cursed Gnomes, as they have not shown up in a single battle since the vault event ended.
Steps to make it happen again
Play Explore or PVP and watch them NOT show up.