My EX is going to my friends and asking them why I left him, WHY is he doing this?

Question

I thought a relationship was between TWO people...

My friends are NOT dating him!

Answer 1

Obviously, he either feels he can't talk to you or that you won't be honest with him.  He's looking for answers, trying to figure out how he screwed up, probably hoping to fix the problem and reconcile with you, and he's going to ask anyone he thinks might possibly have an answer for him.

If you want him to stop, call him and tell him yourself.