If the description of "exploding" and "tearing [a] bag to shreds" are accurate then it has to be a physical modification of the pagers, lithium ion batteries don't explode with a lot of force when they go up.
It is indeed not the kind of explosion I would expect to see from lithium ion. (Those usually are a lot more flame-y at least the ones I have seen so far.). But I'm not an expert.
