Might have reached its end of life. Here, mine dies in winter and doesn't return.

I think it's thirsty. It's not a desert plant, you know, and needs some water. It might survive a brief season of drought, but it won't survive prolonged dry periods.

Rosemary can live for 40 years or more in a warm climate.

I suggest giving it a really thorough, deep watering.

Rosemary likes to be dry. My plants in the ground, about the size of yours, get only the water that comes naturally. We sometimes go weeks and weeks without rain in the summer. I use stone mulch around the base of my plants to keep the interior of the plant dry. Rosemary is one herb that doesn't need lots of extra fertilizer (many others appreciate it, contrary to popular belief). The compost is plenty. The 10-10-10 may be too much. The new growth will wilt when the plant is in need of water.

Did you have lots of rain this Spring before the dry spell? It could be too much moisture damaged the roots and you are seeing the results above ground now that the leaves are calling for water.

It more likely is disease like shoot/stem blight. Excess nitrogen can create conditions favorable to this disease.

In either case, remove all the dead and dying branches and cross your fingers.

Good luck,
-F. DeBaggio