I don't think I've ever seen a reasonable explanation as to why they sometimes do that...but they do, I have more than one double white oriental hybrid that will bloom in summer. One explanation that might make sense is that it is sometimes in response to a stress of some kind, like your heat, that could initiate flowering and seed set, but my own climate is mild both winter and summer so I'm not sure that applies here. Whatever the reason, enjoy the flowers :)

I have a well established H. orientalis that still has intact blooms located on the north side of our home with no direct sun only indirect light. It's been blooming since late February. AMazing!

I have 'Gold Finch' in bloom now. We had a very early hard freeze last winter (8F in November) then a cold spring with late freezes, which may have reset it's biological clock.