It must be the Michigan air! Two of mine are doing that, too. I did the same thing, I put them out for the summer and I meant to bring them in yesterday, but didn't because I fell asleep early (oops :blushing:) and now I'm hoping I didn't destroy the buds.
Anyway, though, I can't say if this is common because I've only had them a couple of years now, but it didn't happen last year, that's for sure. Hmm...
Did you happen to repot yours this year? (I repotted mine in March and it's possible that is why they are blooming. Two of mine bloomed this past summer and one of those two is one of the two that is blooming now.
It's a workhorse!)