Poor casting and manufacturing quality control. Aluminum casting, when done in a mass production setting that is producing low quality consumer goods, is typically done using flash casting. This process is incredibly hard to keep in control. By control I am referring to the use of control charts and statistical quality control techniques that infer the state of the process.
When a mfg process goes out of control there is typically a trend or TTS (time to signal) that lets the engineers know that something is causing the product come off the line with an unacceptable percentage of nonconforming parts. For things like airbags mfg engineers might implement total quality control checks for each airbag and component. For things like your bike frame, they probably insect something like 10 of every 1000 and use SQC (statistical quality control) techniques to infer the state of the process at any given time.
As the casting or tube rolling process enters some state of abnormal performance, from say a line worker throwing trash into the bar stock or the die is worn or not heating uniformly, there will be a situation where the bike frames are getting closer and closer to the predetermined control limits. AKA out of spec. Once the parts are consistently nonconforming then some process control system should signal. Hence Time to Signal. Or Average Time to Signal TTS and ATTS. For consumer goods the ATTS is usually high, and therefore several hundred or thousand parts can be shipped before the control engineers realize there is a problem.
Poor part tracking systems and control processes make it difficult to determine how many parts are affected and it should be assumed that all parts need to be recalled back to the last known time of conforming parts. Bikes aren't missiles, and therefore, some process introduced an air pocket into the rolled aluminum which was then fed through a machine that uses a electroplating typeish process to paint and cure the tubes at a rate of about 800 feet per min or more. Thus, the pocket was covered by the paint (if it was exposed at all to begin with) and over time the pocket worked it's way through. No human ever looked at the frame other than a quick once over I'm sure.
TL;DR Poor manufacturing control systems let impurities get into the AL during casting or rolling. Air pockets are common in cast aluminum. The OEM doesn't inspect every part.