What would prompt an adult to pick their nose and eat it?

I feel gross just asking this question. It’s pretty self explanatory though.

Back story – today I was sitting in my “college algebra” class and the guy behind me tapped me and told me he just saw some kid a few rows over pick his nose and eat it. I thought “no way, no one does that”. Then I looked over, and with my own two eyes, I saw it. This kid was SHAMELESSLY picking his nose, really digging in there, and licking it off his finger. I was in shock. I sat with my jaw dropped and stared. Rude, I know, but this was true disbelief.

Have you ever done this?! Another question – WHAT WOULD PROMPT someone to do this?