guys competing in the workplace for a girl

At work theres a guy who slowly showed interest in a girl (he's possibly an ISTJ). His co-workers (even ones shes worked with in the past) are acting more friendly and peeking in her workroom. The first guy is young enough to be her son, I don't think he knows because she looks younger. Hes been friendly with her co-workers and it seems to be a well known fact amongst some of his coworkers that he's intersted in her. The guy possibly has a girlfriend also.

Can anyone explain what this is?
What could the original guy be saying to stir up this sudden interest in the girl? Also why the effort to get to know her coworkers and be friendly? Any thoughts would be greatly appreciated.