Sometimes one picture tells more than 1.000 words...
Bloodthirst has 4.5 seconds cooldown, just have a look at the time differences between the applications. Each time he hits the ability he has a way higher chance to crit compared to all his other skills (exactly twice as much), netting him more enrage uptime, Raging Blow charges and so on. That's downdraft cascade he's facing due to delaying it by one, often times two GCDs. If he just hits the ability every 5 seconds (which he won't, but to give you an idea) he'd get 68 bloodthirsts out in your kill (340 seconds) - he only managed to dish out 47, measly 70% of what's possible.
If he can't use his rotation properly while moving he should reconsider his keybindings.
But now for the bummer, he didn't use skull banner???! What the heck? It's not only a huge dps buff for him given the 4 piece he's wearing but also a huge dps increase for your whole raid. In fact he only used it twice on all your kills but seven times on trash, i
really don't get it.