If it's in a container, my first thought is make sure it's not pot bound.
2nd thought is could it be fungal infection on the lower leaves, but judging by your location, humidity may not be an issue.
So maybe it's heat and lack of water.
My mint flowers in late summer during the drought, and it's a great bee/wasp attractor -- I once counted over a dozen different species.
The mint comes back with fresh new growths after cooler weather arrives.
In your case, may be cut the flower buds to encourage branching and give more water and maybe move the container where it receives less direct sun?