Two of my peach trees and 4 of my citrus trees are doing the same. Newly planted trees do goofy things, it's normal.

Right now I have flowers on established plum, pear, apple, and mulberry trees. The reason? A severe hail storm nearly defoliated everything and dropped close to 3 inches of precip which is enough to kickstart any plant in my arid climate. I've seen the same thing after severe late freezes here, so I think bamboo rabbit is right that any abnormality in the growth of the plants (such as newly planted trees) can cause this. It's probably even more common in mild places such as Florida.

My young dapple dandy pluot did that last year. Started to go into dormancy with cool weather in the fall. A warm spell in the late fall or early winter caused a few blooms to open. I picked the blooms off and it had a full bloom in spring.

I have a nectarine in a pot that I bought last spring along with a peach and plum. My inconsistent watering and the Texas summer were a bit hard on all 3 trees. The nectarine lost all its leaves by early August and I assumed it was probably a lost cause. Then in early September we had a cold front come through, several nights temps down into the low 60s. Next thing I know the nectarine has little pink buds forming on it. Now 4 weeks later the blossoms seem to be finished and the tree is all leafed out again.

Seems totally the wrong time of year but maybe there's nothing to worry about? I'd already given up on the tree a few weeks ago but after all this blooming effort I'll be sad if the tree doesn't survive the winter.