This is what happened.
Foes Charizard used fire punch
My Zoroark used protect for 3 turns
Foes Floatzel used aqua jet and is supposed to KO my Zoroark, but it said "it hung on to focus stash"
Most likely,if your Zoroark was holding leftovers,it would have gained health while it used protect for 3 turns and its HP might have came back to 100%.Then the focus sash would activate.
If it was not holding leftovers,you might have accidentally given it a focus band instead of focus sash.
There is a 10% chance that when the holder is hit by an attack that would otherwise cause fainting, it can survive with 1 HP. Can also protect against multi-strike moves, recoil damage, self-inflicted confusion damage, and damage from Future Sight or Doom Desire.
Hope I helped.
You are right, Focus Sash is not supposed to do that.
Are you sure you did not give it Focus Band? That would explain what happened here. Focus Band has a 10% chance of working each turn.