That could be it actually, there's a turn limit at which the attacker is forced to retreat, and a couple turns afterwards all attacking units are killed if they haven't left yet. Golem's dont retreat under these circumstances, but just die. I lost some perfectly good snake bladder stick + Golem combos to this, and I'd bet it's what happened to you.
Next time give him some winged boots.
If its a astral golem he can just teleport anyway with +1 astral starshine skullcap, so you dont need the boots. And the lightning bolts arent really needed with that lumbering monstrosity with the unquenched sword.
