Why do ladybugs swarm in the fall?

We live in a log cabin on 10 forested acres Oklahoma. In the last three years that we have lived in this house, we've had an infestation of orange lady bugs; each year worse than the previous. This year after the first cold snap, we woke to find the south side of our house literally covered with the orange bugs-thousands! Even the cats were freaking out. For $500.00 we accepted the call of an exterminator. Best money I ever spent.