digging around FR24 and FlightAware, I find N237AK operated AS698 SEA-PHX early yesterday morning, then made up AS751 PHX-PDX which left PHX 4+ hrs late and diverted to LAX (N462AS continued the flight)
N237AK made up AS1219 this morning; as OP stated, it returned to LAX after about an hour … the flight is now en route to SEA using N709AL
so I’m guessing the jet went mechanical after leaving PHX yesterday, but whatever they did overnight at LAX didn’t actually fix the issue, and it was significant enough that they had to go back there rather than continuing to SEA