Was stood in the Trade of Blades when it happened. Had 4 other players nearby. 1 was a Cleric. 1 was a Fighter. 1 was a Warlock and the last 1 was a Paladin.
Dunno who was responsible for the buff that causes it. But it was one of the buffs visualised in my bar at the top of the image. Don't currently poses the spread of classes required to pinpoint which specific buff is responsible so I'm hoping someone in the community can look at the buffs and list which ones they are.
I will go over and triple check my own class (rogue) and make sure it isn't me buffing myself with 500k Crit Resist.
Other than that image is below showing stats table + buff bar at the time it happened.