Polyethylene terephtalate Biodegradability
It's oxo-biodegradation* with an asterisk. PET polymers are polyester meaning that the monomer units are link by ester functional groups. This ester group can be hydrolyzed in the presence of water to for the alcohol and acid that formed the ester. For PET the alcohol is ethylene glycol which is readily digestible by bacteria and the acid is terephthalic acid which is not as redily digestable but still digestable.

It is for this same reason that poly(glycolic acid) which is used in dissolvable stitches and poly(lactic acid) which is used in food containers (and 3D printing) are also biodegradable.