Well, she asked if he had a grudge against the airline and he replied that he had a grudge but not against her airline...she put the word in his mouth.
As to the literal interpretation of said "grudge," the state of America in 1971 is the very reason I think it's impossible to use it as a reliable investigative tool. You can choose any almost industry a there was a downturn in profits, layoffs, lives being destroyed, etc.
Because the options are essentially limitless, Cooper didn't bring it up himself, and he never elaborates on the exact importance of his "grudge," I'm disinclined to take it further than that he was responding to a question in a conversational manner. He asked for a certain amount of money, tried to give some of it away, and was uninterested in any other specific items save for those used in his escape.
Cooper seemed hesitant to do anything that would help identify who he was; deflecting questions about his past, thoughts on current places, reclaiming items that he had left identifying information on...why would his vague elaboration to Tina about his motive be anything other than another deflection.
I will not say that he didn't have a motive; Cooper was motivated by something to commit so brazen a crime, but money was the ultimate goal. He could have been dissatisfied in a divorce or in some kind of money trouble elsewhere. He could have been unhappy with his retirement from an affected industry or even the military and decided he'd rather have a bundle of cash to launder in Vegas.
tl;dr the grudge is useful only to the point that a suspect has to be at an inflection point in their life in November of 1971 that would explain why they wanted $200,000 so badly. It's something that we can use if we find a promising suspect...but it's a double edged sword because it does little to narrow down the suspect pool at all.