REV man
If I am not banging away at a computer I will GLADLY help anyone. If I am doing complex calculating in my head and working an old fortran computer system at the same time, I might very well get short with you. Sorry, it really is a safety of flight issue.
CRM is all about breaking the mistake chain at its earliest point. The earliest point is flight planning (well some would say bidding...), and something that is sort of taken for granted these days. If a pilot is at a computer and someone interrupts him, it is a flight risk. maybe a small one, maybe a large one. If that pilot is flying Dallas-Austin for the 4th time today a pretty small one. If he's about to roll off 3 or 4000 miles including an ETOPS leg a much greater one.
Sadly there aren't enough computers or remote "Mini OPS" where these things can be done out of the view of the public. There should be, but that costs money. Furthermore Flight planning software should ALSO be in plain english, but sadly the 40 year old computers that run most airlines in the USA (and FAA) are not. So its all fortran or worse.
If the pax had waited quietly when the pilot working the computer was done I'm sure he would have addressed his need in a courteous manner. I don't know any pilot who would not. I do know a few who get short when interrupted in front of the computer, and if they are of the type that prefers not to multi task, far better to be short with the pax, then make a mistake in the flight planning. Dead passengers never come back, everyone else (sadly or fortunately depending on your POV) comes back for the next fare sale...
Frankly if management would spend some more dosh to make sure ALL flight planning was done in an OPS or on your own personal computer rather than in the terminal in front of the pax (A totally unacceptable method) we wouldn't be having this conversation and safety would improve. In this day an age a few pennies a ticket would pay to push equip all IPADs with cellular and push the flight planning software over the air to the IPAD.
And furthermore, I too have been on both sides management, Union and pilot... Customer service is important, but it is SECONDARY to safety.