A drop in hormones triggers a bleed. Think of the placebo pill in birth control - you stop taking hormones and the drop in hormones - progesterone specifically - triggers the bleed.
So what causes a drop in hormones when you change your eating habits? A lot. It's not just one thing. It's eating 3 meals a day. It's eating a balanced meal at each meal. It's eliminating processed foods that are rich in crap that confuse our hormones. It's balancing our cortisol and insulin. For some people dairy or soy contribute as they can confuse our hormones. Ultimately, though, the body is all connected and it's a combination of these and many other things that contribute to helping your body work how it's supposed to, with adequate levels of hormones to support health. A temporary drop in hormones is a sign that your body is adjusting accordingly.