You can figure out the reason that they left by asking them.
This isn't something that is controversial. There is lots of research into this.
https://medium.com/tech-diversity-files/the-real-reason-wome...
http://news.mit.edu/2016/why-do-women-leave-engineering-0615
Are just two, actually published research attempts that I found with a 2 minute google search.
We need some sort of exit poll data for people leaving the tech industry if that exists. We've seen the data for women giving up after receiving fewer rejections than men, but to my knowledge any accusations of toxicity are purely anecdotal.