Was running a bounty with a Demon group and somehow managed to complete Garrosh 14, even though he was not in the group (much less on the board).
It only took two fights to complete Task 14. I had Jaraxxus, Mannoroth, and Gul’dan in the board, farming Fist of Jaraxxus casts.
After the first fight Task 14 counted 12 uses of Horde Strength. Task 14 completed after the second fight.
I’m not certain which skill was triggering the Task completion, but the only skill that triggered multiple times was Fist of Jaraxxus and the “give demons health” effect from Mannoroth skill 1.