Why is my ex suddenly so angry that his cousin liked me on an app?

My ex's cousin liked me on app and maybe the he didn't know since we never met but I asked about it. My ex laughed at first it was late at night. Next day angry and mean saying he doesn't care. I was with him little over a year. Why would I date his family member? Also making sure he wasn't telling other men things about me. I have a right to look out for myself.

Why all the anger? I ended up saying keep ur f'n family away from me and blocked him.

Why is my ex suddenly so angry that his cousin liked me on an app?
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