We have a pear tree that is producing ripe fruit now (it is early-September). The tree is doing what I've never seen before: There are blossoms on some of the branches! Is my tree dying? It looks so healthy, and the pears are delicious. Why is it doing this?
Our old Bartlett pear would occasionally bloom in the fall, here in zone 6b. It is generally caused by weather conditions tricking the mature dormant buds (for next season's growth) into breaking dormancy in the fall. Usually this doesn't hurt the tree health-wise, and can help you with fruit thinning the following year. The time in fall that they sometimes bloom coincides with the time in spring that they bloom, as far as day length is concerned. The other factors, such as temperature and moisture, have to fall into place at just the right time for this to happen.
It's more common on older trees, which grow for a short period in the spring (young trees often put growth on new shoots well into the season), because this wood has matured further and the buds are ripened and ready for the next season.