Okay, lemme preface this with - Spot is our almost 11 year old JRT. He hasn't done this particular behavior in YEARS.
When he's in a reallly playful mood or wants our attention he finds the nearest piece of plastic & starts ripping/tearing at it; growling & barking all the while.
He used to literally RIP plastic grocery bags into shreds - leaving him with plastic between his teeth.
It is for the attention I think, not sure though. Anyway, it always makes us laugh because he looks like such an idiot.