Whay = typo for "What" I think.
Estrogen is the main culprit, but no one really understands all the mechanisms and interactions within the body that cause gyno.
Wanna limit your chances?
1. Don't be fat. The leaner you are the less prone you are to estrogen sides.
2. Don't have pre-existing gyno. I know this one you don't have control over, but it's true.
3. Anti-e's on hand, especially letro/femara. It smashes estrogen so well, many users have reported it preventing gyno deca.