Why are these flies spinning on their backs?

Over Christmas and New Year I was in three different houses over a three week period: my own, my family's and a friend's. In my own and my family's house I witnessed a (single) fly flying past erratically, landing on its back and spinning at great speed. In my friend's house I heard my friend's husband wonder out loud what on earth a fly was doing - "is it spinning on its back?" I asked him - "yes".

What's this all about?

Are the flies trying to right themselves, recovering from a stunning injury, trying to free themselves from something, dying (seems awfully effortful, poor things), spinning for the hell of it or having some other neurological problem?

After a chat about it on Twitter it seems that it might be the effects of a pesticide that's causing a neurological effect. Perfectly plausible although none of us in the houses mentioned were using it, however flies do fly and they may have been sprayed elsewhere and arrived later at our respective houses in their agitated state.

A few months ago I had a wasps' nest sprayed with a white powder via a puffer. Shortly after the powder had been puffed on the nest the wasps left and swarmed outside it before dropping down and twitching (slowly) before dying.

This thing with the flies is very strange to see, I've never seen anything move that quickly under its own power - as you can see in the videos below it's quite a remarkable performance. This is just a selection of several videos on YouTube showing this phenomenon.