Assuming you are transplanting them from their nursery pots to a container or the soil, there is a phenomenon called “transplant shock,” which may occur shortly after you transplant a plant into a new environment, especially if the new environment has very different conditions (more light, more heat, less water, etc.). The plant may drop its flowers or some of its leaves in order to prioritize producing new roots. It sounds like this is what’s happening with yours, made worse by the heat of summer. After it is planted and settles in, the phlox should produce new flowers. Do yours? 

Alternatively, it could be that the phlox is not used to so much sun and heat. It may have lived a more sheltered life in the nursery, so perhaps try keeping it in partial sun for a week or so when you first bring it home, then move it into full sun.

Apparently there are some pretty tall phlox varieties: Border phlox Phlox paniculata “David” is one; Mile High Pink Phlox is another; Caspian, Hesperis, Kirmeslandler, Old Cellarhole, Robert Poore, Russian Violet are others. Heights of these mention range from suggested 5 feet to about 6 feet. It may be that you have perfect conditions that inspire yours to reach even higher! As for how they got into your garden…an animal or bird may have transported the seed but it’s impossible to know for certain.

You should be able to achieve that effect, if the conditions—soil, sun, and water—are appropriate. Take note of the variety, ask questions about height, and you will not be disappointed. There are many varieties from which to choose (“ground cover” will not give you the desired effect).

You should have no problem. Phlox are perennial. Take note of the guidance above and be prepared to give it time to settle in.

The general guidance for propagating phlox seems to lean toward stem cuttings dipped in rooting stimulant powder.

The Virginia Cooperative Extension explains in detail:

Generally, tip cuttings are easier to propagate than root cuttings. Select second growth of phlox for cuttings. Make tip cuttings 3 to 6 inches long. Treat the base of the cutting with a root stimulant. Leave all foliage on the cutting except the part that will be below the soil line. Insert one cutting per peat pot. Place peat pots of tip cuttings in a lightly shaded place. Cover with a sheet of clear plastic. Check regularly to make sure the cuttings do not dry out. When cuttings do not pull easily out of the soil, they have begun to root. Make holes in the plastic sheet to increase the exposure of the cuttings to the air. This will harden the cuttings. Every few days, enlarge the holes or make new ones.

You can also make root cuttings of phlox. Dig the plants in late summer after they have bloomed. Select pencil-sized roots; cut them into 4-inch sections. Put each piece in a peat pot. Prepare a tray of peat pots as for seeds, except the soil mix should be 2 parts sand, 1 part soil, and 1 part peat moss. Water thoroughly.

This just happens sometimes. Is your phlox getting enough sunlight? It needs 4 to 6 hours of DIRECT sunlight at minimum. As you mentioned, you can also fertilize. Applying Miracle-Gro one time per month should be ample. The only other thing we can think of is crowding. And we have read that lack of pollintors can be a cause.