Why do bubbles form on the side of a plastic bottle?
Your reasoning seems good. A number of things can affect nucleation of bubbles, such as surface defects, which you state, as well as temperature (was one side of the bottle facing the sun, perhaps?), and even sound.

A fun demonstration is to put a cup of carbonated beverage in an ultrasonic bath (e.g. parts cleaner or humidifier). Here, cavitation serves to nucleate bubbles.