Mine did that few years ago. Out of the blue in August. It was rather sparse but still very much outside of its regular blooming season. Strange indeed.

This is not uncommon with a variety of spring blooming shrubs and trees - rhododendrons, azaleas, forsythia, magnolias and various viburnums can all bloom sporadically out of season. Mexican orange, Choisya ternata, is usually always good for a light late summer/early fall flush of flowers. Typically it is a reaction to stress, usually heat or drought stress. When faced with stressful growing situations, the plant enters an early, pseudo-dormancy and tends to shut down growth. Once the stressful conditions are relieved - temperatures cool down or rainfall or irrigation is plentiful - the plant 'wakes up' and its biological clock tells it it's spring and it will produce flowers. Usually just a few and nothing like a normal spring bloom cycle. And it doesn't seem to overly affect its normal spring bloom.

Fundamentally winter or early spring blooming shrubs that may bloom ahead of schedule in late summer or autumn do so because their otherwise over-wintering flower buds develop and mature so early that some of them open prematurely. Any buds that pop out of turn are buds that won't be opening at the normal time later.

This is not the same as flowering at the tips of new growth the same summer as the growth was made, after these shoots mature a bit. Magnolia liliiflora hybrids such as saucer magnolia often do this, and are perhaps somewhat unusual for both setting over-wintering flower buds that open the following year, as well as flowering on the tips of new growth in summer. Unfortunately these summer flowers are usually underdeveloped and aberrant looking.

Also, some rhodies tend to rebloom in the Fall more than others. Elizabeth and Elizabeth Hobbie I remember doing that every year.

'Yellow Hammer' also. Such rhododendrons are all early bloomers that bud up well before the main flowering season the following year.

I still have a 'Yellow Hammer' I acquired over 30 years ago. I'll be sure and keep an eye on it this year. The blooms are small and easily missed.

Mike