The reason I added the "if" is because I didn't see any information about age and don't know the specifics of the engineering/specs. Bolts needing the be checked annually and tightened every 5 on average could be perfectly reasonable with how much stress is on airplanes. There's a reason frequent inspection is enforced more heavily on airplanes, and it's not just because failures mean potentially falling out of the sky.
But yeah, it's entirely possible they fucked up, but it's for sure United Alaska did.