One STD could rise in incidence while another falls either due to changes in antibiotic availability or disease resistance. One could have developed antibiotic resistance while another may have a new antibiotic defeating it. Further, there could be changes in the sexual habits of different populations which raise or lower the incidence of one or the other STD.
Work Step by Step
Consider: there is a differential change in the incidence of diseases, and the two things which could vary are the effectiveness of the bacteria and the sexual behavior of the populations affected. Then, based on that, think of ways that one bacterium could cause more infections and the other bacterium fewer.