... the motive for taking legal action is doubtful ...
In a word, contingency. It's the lawyers who are out to make money.
Ought to be easily defended by AA/UA, who were in compliance with all FAA regulations in force at the time. However, careful selection of a jury with anti-airline sentiments may easily result in a different verdict. Remember in the US,everything is someone's fault - s*it doesn't just happen there.