Often there's the implication that they're not local and have come from elsewhere (perhaps?)...
Even walked there of their own accord :-) crossed the road for a midnight stroll, or wandered down to the water for a drink (it was believed that the Carnac stones went to drink in the Bay of St Colomban on Christmas Eve).Perhaps such stories indicate a folk memory of stones being moved from one place to another (and sometimes washed for medicinal purposes?).