I've seen similar results in OS versions. Release date for iOS 5.1 saw about 2 percent of users running our app, over the next few days it started to overtake 5.0.1 as the dominant os.
I would say it really is down to OTA OS updates, and I think it is a great thing, as it means that you can potentially drop support for older os versions far quicker than previously, and this also means you can start to optimize for new os versions and support functionality that the new os versions bring.
As for why people are still on older os versions, I'd say it's probably because they are unaware of there being an OS update. Think about your non-tech savvy aunt / uncle / grandparent (or even parent!) and how they will just use the iOS device and probably not worry about app updates, let alone updating the OS.