They don't know the security theater procedures so they take longer to go through them and hold up the people behind them (laptop out of bag; liquids, gels, and aerosols in plastic bag; etc).

(That's one of the advantages of TSA PreCheck - not only are there fewer people in line, but the people who are in line are frequent flyers who tend to move fast.)

"What do you mean, does this big sign that says 'take off your belt' mean that I actually have to, you know, take off my belt? And this form that says 'are you carrying any fresh fruit on you', I didn't think that would apply to this orange in my backpack because, you know, I didn't see anything wrong with it!"