Hi Maggie, Yes its normal if the weather stays mild and sunny in the autumn, to get a second bloom but its not usually as good as the one in June. I've seen a lot of them have big ripe pinkish pods this year as well!
We have a magnolia stellata (which has the little white star-like flowers) and that too has had the odd second flush. It has happened in one or two years, normally as I seem to remember when we've had warm late summers with plenty of rain.
We've had a few bushes this year that have had a second bloom, although it's nice to see, you worry about why it's happening! global warming and all that as it seems to be happening more often over the last few years.
Put it down to the weather. I hauled all my nasturtiums out three weeks ago, and they're all coming back with new growth - in October!!?
I'm in a potato-growing area, and this is the first year that anyone can remember when crops have been harvested without there having been a frost.
Global warming indeed.!