My guess was a tainted thickening agent, I doubt it was put in maliciously, there was no top hatted man twirling his mustache cackling "haha now we will put anti freeze into our cough syrup and kill the children with it" but more criminal indifference as the batch of glycerin that they got for a really cheap went through a non-existant quality control process that would have found it tainted with the ethylene glycol produced at the same chemical factory.