Why would someone accuse me of staring at them when it's my job to make eye contact & greet them?

I am a store greeter in a large & busy department store. The store training included body language, how we are supposed to smile, make eye contact and greet each customer as they enter & exit the store. Yesterday a customer yelled at me and accused me of staring at him each time he entered the store. The other customers entering the store got very quiet and watched to see if he would get violent. I only remember seeing him once before. What could be the reason he acted this way? Most customers either smile back, reply, or look the other way if they don't feel like being friendly. And it wasn't as if I was focusing and honing in on him and neglecting the dozen or so other customers entering the store. I have been working at this job for 3 months now and this has never happened before.