Old-World Golem

When Old-World Golem is summoned, tap it. It does not untap unless the owner declares it 'untapped' on their upkeep step.
At the beginning of each upkeep step, if Old-World Golem is tapped, grant it +1/+1.
The old world was harsh, but those who survived it were beyond cruel. I'm assuming these were kept to stop their cruelty from returning, but if so, why has it started moving?