Swarming ladybirds

Ladybirds are a cherished friend of the gardener - they munch the greenflies and blackflies that wreck our crops.
I am very lucky in my new garden because I have plenty of ladybirds of all varieties so my salad crops have been bug free.
However, in the last week or so, I have noticed a marked increase in the number of ladybirds, with many flying into the house.
With a little research on the internet, I have read that they can "swarm" in the autumn.
Last August, ITN reported that millions of the insects swarmed in Somerset and Norfolk, where cars were covered in a "thick carpet" of ladybirds.
As it is already October, I'm hoping I won't have a ladybug plague of my own but I'm intrigued to know what's going on.