I've heard a lot of theories about what is causing this, the most common being plastics, increased sugar consumption, more people worldwide being overweight/obese, more people worldwide being sedentary/less physically active, exposure to various harmful chemicals, and more. I'd assume it's probably a combination of several factors, but similar to plummeting male testosterone (which likely has similar and potentially related causes) I don't see this as likely to get anywhere near the amount of attention and funding from various governments and organizations around the world as it would if it were affecting female health.