Yes its all because of crit percentages. Even with a high %, theres a possibility to miss a bunch of crits in a row. My shamans have like 65-70% crit buffed during this combat (with elemental mastery and trinkets etc). Usually I don't get one that is so far below the rest though (thats why I said it was bad luck).

I'll have to remember to keep screenshotting, maybe onetime I'll get one with all 4 shamans > 5000

And yes, for these screenshots im using the:

FS, LvB, LB, LB, LB, LB, LB, LB, LvB, LB until dead method (so many LBs because bloodlust is up, LvB is still on cooldown most of the fight). I use bloodlust and trinket before I even pull the boss so I don't have to stop DPSing to do that. Also use boneshield pre-combat and then hit icebound fortitude immediately after pulling so I don't have to heal at all.