Belfast International Airport was "blind-sided" by Aer Lingus negotiating with its main competitor, the High Court heard.
A boss at Aldergrove said management believed there were still five years left on a partnership with the airline when it decided to move its Belfast base.
BIA is suing Aer Lingus for £20m damages over the switch to George Best Belfast City Airport in 2012. Proceedings centre on the terms of a deal said to have been reached in June 2007.
Lawyers for BIA contend that a letter from the airport's former managing director to Aer Lingus contained the terms of their arrangement.
But the airline claims only the price to be charged per passenger and launch support was involved.
It never committed to keeping a base at Belfast International for a set period of time, according to its case.
In court, BIA Business Development Director Uel Hoey said: "We had a five-year partnership with Aer Lingus and fully expected another five years.
"We had been effectively blind-sided by a discussion which was taking place in isolation with our competition."
The case continues.