Creepers are exploding in the night when no player is around.  The first few times I thought it was the other players accidentally triggering the creepers.  But this happens when I play alone on the server as well.  I have been landscaping and trading with villagers and our base is close to it.  Almost everyday I find some building damaged or a large creeper crater in the town when no players were in the area.

The only thing I can think is that the golems are attacking another mob and hit the creeper as a result.  I've never noticed this happening before.