I'd have to agree with u/chronicpainprincess on this one!

A lot of ingrown hairs have indeed been either cut or shaved, and gain a "pointy" micro tip.

They then borrow back into the skin, and invade downward, carrying bacteria down with it.

And then the skin heals over fairly rapidly (since it's only a micro sized incision made by the hair tip) thus trapping the hair and bacteria inside, and then a warm/moist bacterial friendly environment begins to take hold.

Same process can happen with someone else's hair stabbing into you, as u/chronicpainprincess noticed in her work. In which case that's doubly gross! Because it's someone else's nasty bacteria being inserted into you. (Yikes!)