They are moving to whomever has the best deal for replacement aircraft.

They made a great business deal as the North America launch customer for the A350, and an even better deal converting some outstanding 350 orders to the 339. As a customer and shareholder, I love that they did what they could to accelerate the modernization of their widebody fleet.

If they are "moving" to an all Airbus widebody fleet, then the article should quote someone in management who said that was in their long-range plans. That this is a conscious decision. That quote does not exist. What does exist is evidence that they make the best deal on replacement aircraft that fit their needs and retirement/expansion timelines.

It is the simple difference between Association and Causation. Flip a coin 3 times and get heads does not change the probability the next time you flip the coin it will also be heads.