Not unheard of for azaleas to throw a random bloom or two late in the season, I have some of mine doing the same. There are a few that even bloom more than once a year. Really nothing to be worried about, just plants doing plant things. Btw thats a really nice flower you have there, good luck with it!
I've had it happen three or four times, usually shortly after they arrived from somewhere else. Might be the change in location, temp, humidity, etc. provoking the reaction. I've always looked at it as a bonus.