You do it, too.
Curiously, I've argued with plenty of people about this. I've watched them do it and later pointed it out, only to hear them deny that they did it. Then they say "oh, well the waitress didn't set it down with the way I wanted it". Mind you, many of these people will simply turn the plate a centimeter or so, but they still touch and adjust it, but usually aren't even aware that they or others are doing it.