The bacteria in plaque produce toxins (harmful substances). Toxins surrounding gums and periodontal support tissues (fibers, etc..) break down and tooth and gum gradually move away from each other.
And periodontal disease develops  in this way, so unless cleaned and treated  periodontal pockets get  deepen even further.
This time bacteria holds root surface-more vulnerable then the enamel-  surrounded with cementum and  bone support of the tooth is also lost, by swinging the tooth may be lost at the end .