Yeah, without seeing an actual breakdown, it's hard to gauge what went wrong. Bad rotations with constantly overlapping Sens, not keeping buffs up, not keeping Higanbana up, not using Kaiten or spending Kenki wisely in other ways (like to get back to boss quickly after an AoE).

There a LOT that can go wrong, though if he was doing THAT poorly he'd have to be doing a lot wrong. Its possible the fight mechanics got him so discombobulated that he couldn't keep the rotation going properly. I once saw a SAM who just couldn't adapt and he would start every rotation after dodging with the Kasha rotation, even if he already had the buff from Shifu and the Sen.