What do you mean why? Because he's a filthy scumball that's why! Imagine getting his family to reach out to you too. What a gross excuse for a human being.
Sounds like something a narcissist would do. Using his family as flying monkeys to get you back. Next thing you know he'll be trying to use your family and friends.
Op this isn't like a one night stand that could maybe be explained. Its not a mistake. He actively lied, cheated and fucked you both over. There's no reason other than - he's a total slimeball.
Block him and his whole creepy family.