I suspect the origins lie in the occasional determination of our cousins to use as many complex words as possible, instead of concentrating on an economy of phrasing. Examples include:
1. "What is the beverage of your choice?" meaning "Would you like a drink?"
2. "At this time..." meaning "Now"
3. "Federal regulations prohibit...." meaning "Please don't"
4. "Overhead Locker" meaning "hatrack"
5. "We are experiencing an ongoing delay situation..." meaning "we are running late"
And so on.
It's all part of the fun when flying with US carriers. Don't worry, they think we in the UK are all as crazy/mad as a bag of ferrets anyway, so it cuts both ways!
P.S. - I assume "deplane" relates to anything else that we do "dee", as in decant, deforestate, defenstrate & decompose. I stand to be corrected.