I admit it is only a speculation, but I made a link between the phytoestrogens and some health problem I had.

I had a scare with a autoimmune problem, but luckily it was benign. However I learnt that estrogens weaken the mechanism by which the body learns to avoid fighting itself, and that it was explaining why women had a lot more of some kinds of autoimmune diseases than men. And the missing puzzle piece was when I understood that my autoimmune problem started a few years after I discovered Japanese food culture, and as I had always been in fond of soy drinks, soy creams, tofu, etc, when I was in Japan I was drinking/eating/dreaming soy every meal of every day (this for a few years).

Are you sure about they are lowering estrogens level?

To draw you the picture, I was drinking tonyu (soy drink) all the time so there always was some in my stomach (except when I was sleeping). Maybe the mechanism by which phytoestrogens lower blood level estrogens works only when you give it time to overcompensate after your last intake.