It's because they have an extension cord running over to an outlet in your house. Ha! I'm kidding. Honestly, I'm not a power guy, but I would not expect that to happen. I would give your power company a call and ask them about it. These days, many accept questions online if you want to avoid the hassle of a call. Ultimately, that seems, to me, to be an issue where the transformer feeding your houses isn't up to the task.