I’ve had some insist I know “something”, I.e. the first one, when I was very young (old enough to say what was going on but too young to understand how other people thought!) insisted I knew why my brother bullied me. Like, no, I have no idea. She was really aggressive about it and I found it scary! I only remember seeing her once.
Last one I saw seemed adamant I knew why I disliked my ex, while I honestly couldn’t come up with anything. Fair enough, I do know why now, but I was still in a bit of a fog then and it was only 18 months after I’d split up with him.