Why is a former high school friend suddenly ignoring me in college?

For context, I'm at college, there's this guy I went to high school with that also landed up in the same College as me. The firs few times (twice) we ran into each other at college he greeted me and acknowledged me. Thereafter, everytime he would see me, he wouldn't even make eye contact and randomly just pretend as if I'm not there. I actually find this to be such weird behaviour. Oh and he's not the guy from the question I posed before this.

Why is a former high school friend suddenly ignoring me in college?
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