Talking to my Ex’s friend

About a week ago my boyfriend of 3 years broke up with me. It has been really hard getting through this and he still talks to me sometimes as a friend. The other day I Snapchatted his friend when I sent streaks to everyone on my snap to try to start new streaks. We’ve been talking just casually since then, not as anything more than friends. I’m not flirting with him or anything and I don’t have any intention to. Anyways my ex found out that we’ve been talking and he got super pissed off and was saying that he’s going to fight his friend and everything over it. But what I don’t understand is why does he care when he’s the one who broke up with me? That just makes no sense to me. Like I said I’m not flirting with his friends or talking to him as anything more than a friend it’s strictly casual talk so I’m just confused. Am I missing something ?