A girl's boyfriend started has been acting weirder and weirder the longer I've known him, why?

I work with a girl whom I am very close with, we actually flirted for a while and she tried getting me to hangout before I found out she had a boyfriend and he started working with us. Ever since then he has been acting stranger and stranger.

It started out with him starting conversations with me out of the blue and mentioning the girl almost as if to let me know that they are together. He shows up sometimes randomly on shifts we work together to bring her a drink or food and then just stands around for an hour.

He randomly bought the exact same chain I have one day even though he had never had one before that. I also found out he asked her why she started listening to Kanye West so much (my favorite artist) and from there, found out he brings me up fairly often. Just last night at like 4 in the morning he added me on Snapchat

Why is he acting this way? Does he see me as a threat?

A girl's boyfriend started has been acting weirder and weirder the longer I've known him, why?
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