Couple reasons.

You could just the first one written up so far. Maybe there were others written up at the same time that you just don't know about.

Maybe there were reports from your fellow overnight associates about safety protocols not being followed that led your store manager to take notice in the first place.

One day I was called by my supervisor to watch a customer who was seen stuffing tools into his backpack. After watching him do it myself, I called the local police department and told them about the guy. The cops arrested him and found over $1000 worth of tools on him. I didn't realize at the time we weren't allowed to do this. I got put on a final.

And as long as I don't do it again it's no problem. Same goes for you.