Why would a guy interrupt a conversation and introduce himself, if you already know him when your talking to some other guy?

I am a female, who went to a staff party. When I arrived at the pub I got drinks, I noticed that my coworker glancing at me. After I got my drinks and left, some other guy started a conversation with me then the coworker walked up to us. He interrupted the conversation and introduced himself to me first and shook my hand smiling then introduced himself to the guy and didn’t shake the guy’s hand. After I finished the conversation with the guy that was talking to me, I was going to head home when I was interrupted by my coworker again. He said goodbye and it would be nice to see me again, giving me another hand shake but seemed prolonged. We stood there holding hands for a couple of minutes while he was staring straight into my eyes. I slowly took my hand away confused why he didn’t let go of my hand after the handshake. Am I reading too much into things or is it possibly that there was something else going on?

Why would a guy interrupt a conversation and introduce himself, if you already know him when your talking to some other guy?
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