Why would a guy reconnect with a girl after years of not talking to her?

Hard to say without more details. Are we talking guys you dated and split up with or just random men in your life? Do you live in the same town you always have or have you returned after an absence? Are you involved now? Have you just split up?

I was involved with them during my teenage years and now I am in my early 20s. I just got back after an absence from being away from college. The shortest was two weeks and the longest was four years.

Congrats on school!
Now. You are in charge. No confusion- talk to whom you wish.
They have obviously heard that you were back home. Isn’t it nice some people want to reconnect? So… Enjoy seeing whom you wish to, and just enjoy yourself.
Politely, yet confidently, decline any invitations from the gentleman you do not wish to see.
And always, always, remember that there are plenty of fish in the sea that you have not even met yet!!!!

I’m betting that your return from college has reconnected you to both males and females from your past.

My guess is that you’re dwelling on the guys because you’re single and in your 20’s. You probably kept up with the guys less than the girls, because, well, that’s just how life works.

I suggest keeping all the reconnections casual. If you can arrange to get a couple of guy friends and a female friend or two together at once, that give you more of a chance to get a read on the situation. Enjoy yourself and avoid 1-on-1 encounters with guys where it could turn private, unless it’s someone you’ve kept up with over the years.

How did they get your phone number?

Hoping for a booty call? Just a guess, given their age and all.