Abusing the sky
I simply don't understand why does it cause so much grumpiness when having to show your boarding card. It'll take 5 seconds to do so and it's part of your agreement with the airline (i really wish people would read Terms&Conditions; trust me, it's all there)
If you wish to understand this, try taking a foundation course in psychology and gaining some insight into personality preferences, behavioural traits and loci of control etc. There are some good ones available by distance learning.
Add to that your familarity with procedures and others lack if it and I'd say things were pretty well setup up for some classic misunderstandings.
None of which, in my opinion, excuses rudeness, but it does help to explain it.