The most common reason for that would be drought. Has anything about your watering changed?
No. They are usually very drought tolerant. This summer has been a little milder than last year. One friend suggested cutting them back but I'm hesitant to do that at this time of year.
I think you could try cutting them back by no more than one-third and then applying a light fertilizer (make sure it is not high in nitrogen).
In my experience lantana like to be fed. The large Miss Huff in particular likes feeding 2 - 3 times each summer.
Thank you. I will give your suggestions a try.