I really think it's a natural progression and happens to everyone. There's a filtering that goes on as you get older as your priorities, pressures, work life, social needs change etc.
My experience was that I realized at some point, I was hanging out with certain people just because we'd grown up or gone to school together, it was really proximity more than anything else. I started to realize that I probably wouldn't be close friends with these people if I met them now and not that they're evil or dumb (etc) just that it's comfort/nostalgia/familiarity that keeps you in contact. I find it a bit odd when people are mainly hanging around with people they went to school with 20 years before. Kinda if I wanted to be in touch, I would.
For me, the tightening of the circle happened a bit earlier as I moved countries (twice) and that had a filtering effect; you only really stay in touch with the people that you want to. Of course, made a lot of new friends and now I'm in my 30's, I probably have better and closer friends than I did 15 years ago, albeit far less.
It also comes down to where you are in your life, and sometimes you're just so far away from where you used to be, when these friends made sense, that you drift.
For instance, I'm not going to get shitfaced on a Wednesday night with a bunch of people when i have an meeting with my employees the next morning. It becomes increasingly difficult to be on the same social plane as friends who have 9-5 jobs, completely free weekends and don't even think about work when they're clocked out. By the same token, some of my friends had babies years ago, and they're just in such a different place. Their life is understandably nappies and early night and early rises, where as I don't get off work until 8pm. I make an effort but again, it's difficult to be on the same plane. there's also those friends that are just constantly drama, it's how they're wired and aint nobody got time for that once you're 25.
I don't give a shit about a big group of friends - I learned a long time ago that even your closest friends can be fickle when it comes down to money or doing the right thing - I just like having a few people that I can really rely on and I see a future with, and we're heading in the same direction.