Excerpt from this article:
That hatred can be explained in one phrase.
“Unexpected item in the bagging area” is a shared cultural reference like no other. It is recognizable by demographics so broad, the only thing that connects them is that they have at one point attempted to buy something at one of the nation’s largest grocery stores, pharmacies, or fast-food restaurants. It is fuel for memes, and tweets, and Reddit threads. It is the worst phrase known to retail. “Unexpected item in the bagging area” seems to be passive-aggressive code for “are you a shoplifter or just stupid?” and it haunts dreams.
Not everyone is trying to be a criminal mastermind. Anecdotally, a lot of people steal from self-checkouts simply because they get angry that an item won’t scan and figure it’s not their job to try that hard. Others steal small things here and there because the absence of a human cashier and the presence of only an obnoxious machine owned by a giant corporation turns it into a crime in name and not in spirit.