How to build relationships with colleagues as new faculty member?
Solve the social things by being proactive. Go to your colleagues individually, after committee meetings, or during their office hours, introduce yourself as the new guy, and offer to buy them lunch or a coffee. If colleagues go to lunch after departmental meetings, politely see if you can tag along. The "somehow" in your question is indicative of someone who did not look for such opportunities or dodged invitations. Lunch is probably the easiest time for colleagues in a work setting to begin their socialization. Everyone eats and almost everyone like some company. 

As best as I can tell, there are only a handful of things that bring faculty members together for forced work, e.g. departmental committee meetings. That being said, departments have social and semi-social functions. Attend as many as you can. Volunteer to help with student move-in at the dorms. Volunteer to be a marshal and graduation ceremonies with other faculty. Find a hobby and check out the university club (exercise oriented ones like cycling, rowing, and running tend to be popular). 

Solve this problem like it was your PhD. Set goals and be deliberate. Also, be friendly.