I’m currently in a Max Raid Battle with a Vibrava. I was typing, and I looked at my switch a few seconds later, when the Vibrava was using Boomburst, and somehow it got a critical hit on every Pokémon. What happened?
This is either just really, really, really, bad luck, or Vibrava used Laser Focus (it can learn Laser Focus at Level 1) on the previous turn, guaranteeing the crit next turn. It also could have used Focus Energy (Max Raid Pokemon in the Wild Area can have TR moves) and got the lucky crit chance on every Pokemon.
Hope this helped!
The most likely answer is pure, blind luck (or lack thereof). The approximate chance of this happening is 0.000302373843%, if my calculations are correct.
Hope I helped!