It sounded like wavy rather than bubbles from the description...so it is typically from mounting issues as as described, or from plies with uneven tension, so as to produce areas of sidewall with tighter and looser ply sections.

As the tire rolls, the hysteresis impact builds due to the uneven tension, and premature failure, especially at speed, can occur.

For normal/low speed driving, it rarely has an impact.

If its a ply tension defect, it was likely present when purchased, and should be taken back.