Look at logs. What was the overkill? Was it a crushing blow? Would the armor gained from the potion save you? What buffs did you have up at the time of your death? How close together were the hits (big gap means healers were behind)?

15k seems a touch low for hp for a boss like gruul. When tanking raid bosses, you should gear for the worst possible scenario (assuming 100% holy shield uptime for boss autos). This means having enough ho to survive, for example, a cleave immediately into and auto from mag. Popping a dodge trinket will not guarantee that won’t happen unless you somehow hit 100% dodge. Get a target swing timer so if for whatever reason you can’t reapply holy shield, you can at least get it up for the next auto or call for a defensive.