They want YOU to be miserable after break up, AND you going merry on your way does not fit their personal narrative of a nice/perfect guy they are.
What they want, what they are desperate to see is You being miserable, unhappy, and regretful. CAUSE YOU DARED TO BREAK UP and even worse -YOU MOVED ON. It just conflicts in every level with the lies they tell to themselves to boost their audacity.
So what they do is to periodically check up (mostly online since it is easy now) all women they had in their lives as GF, crushes, those with whom they were flirtatious, hooked up, etc etc. (yeah even high school crushes)!
If they find that the woman is having a great life, happy, merry and so on - they will do either on of the below ( but mostly first):
Hate, belittle her personality, life, achievements - either publicly or in their head;
Obesses over her and try to sneak into her life in some way, SO THEY stalk even more to find the perfect timing to catch her in the moment of weakness.
That's why ladies I would highly recommend you to keep your social media presence in positive light, ALWAYS. Never text back the trash that once you left rotting outside. Believe me it didn't magically start smelling roses.
In my personal experience I have witnessed how men that are blocked create fake accounts and check out all my IG stories. Sometimes they got courage to text and guess what? They get block and deleted. But they obsessively come back. It's so funny to see how from one time to another the content of messages change from: 'I missed you/loved you' - to passive agressive: 'You are a whore'.
Scrotes truly can't live in a world where they are not the ones having a 'main character' life and everyone else is conveniently living what they believe they 'deserve'.
But at the end they are the pathetic ones and who will DIE MAD.