I think they arranged a secret meeting and got sloppy. They also most likely figured the media wouldn’t run with it if they got caught (or at least that they could control the media)
I think that’s the most likely possibility.
But giving Lynch the minimal benefit of the doubt, she did have a reason to be here that day, to speak at a conference. Her schedule is open to the public, Bill could have looked up her schedule and decided, in his bull in a china shop way, to fly to Phoenix and “just happen” to be on the tarmac with her, to personally intercede on behalf of his wife. (He’s probably done the same thing effectively to connect with women he wanted to form other kinds of relationships with, “just happening” to be in the same restaurant or hotel.) Bill has used his personal charisma, which shouldn’t be underestimated, to influence people for years and could have thought he would be able to schmooze her as well. If that’s the case, I can imagine Lynch seeing Bill glad-hand his way up the ladder into her plane and thinking, “Oh, &%@!!!”
Not only is Hillary the subject of a criminal investigation by the FBI, so is Bill - part of the FBI’s investigation involves potential public corruption and influence selling through the Clinton Foundation, of which Bill is a part, and Trexit, the public relations firm he was instrumental in starting.
Lynch is not an idiot - if this was not collusion between both parties, she had to know this violated legal ethics by creating at least the appearance of impropriety. She should never have let him on the plane. If this was Bill trying to illegally influence the AG, his actions significantly reduced her ability to operate, and put a greater public spotlight on the case and her ability to decide the case “lacked prosecutive merit” or “didn’t rise to the level of a felony.”