UA, at its sole discretion (with some possibility of review by the DOT/BTS/etc I believe), determines and publishes the reason for its delays or cancellations.
...
If it's critical that an airline be absolutely exhaustive about the dozen potential "whys" of every flight cancellation or delay, then that should be written into the legislation / policy. I'd love to see the airlines have to interview 10 different people to write an essay on the cause of every single delay, and then be 2nd guessed by passengers who know better.
I think even you would agree there is a WORLD of difference between "the flight was diverted because of a mechanical failure" and "We're going to claim it was a bird strike publicly, just so we don't admit we almost killed everyone on board due to negligence."