I don't know the answer but I used to drool a lot in my sleep and after taking several weeks of quassia and doing a Liver Flush a big tangle of Tapeworms came out. I haven't noticed much drooling since then.
I've wondered the same thing, myself. I can only make vague speculations. Salivation is essentially a digestive function, and as Tapeworms typically reside in the digestive tract, the distress and imbalance they cause there may somehow result in drooling.