As the saying goes, never attribute to malice what can be explained by incompetence.
In the case of United Airlines, be prepared to adjust your threshold of expected incompetence dramatically upward; since their merger with Continental multiple years ago, system-wide outages have been an every-couple-of-months thing.
Edit: Now the Secretary of Homeland Security is saying that there is no evidence of United or the NYSE being due to malice.
Edit (about an hour later): Now the White House Press Secretary is reiterating the point about no indication of malicious actors.