My thoughts:

1. Some people are just rude to everyone.

2. Some people feel that staff in the service industry must be talked down to because in their view they are:
- stupider than them
- of a lower class / social strata than them
- 'that's what they are there for'
- all of the above

3. Some people think that the way to look important / rich / clever / all three, is to talk down to people.

4. Some people are frustrated at being talked down to all day by their boss and so repeat the poor behaviour by taking it out on anyone they can.

I like to call it the 'Middle-aged, middle-manager, mid-life-crisis, get me Willie Walsh on the phone now, I'll never fly BA again" syndrome.