Actually, that's what I suspected in my first playthrough, but if you interrogate the bodies of the tollhouse members they say that it was goblins and gnolls, not the Knights of Tyr; this is backed up by the journal entries in the tollhouse worker, which you find in the basement; both were preparing to abandon their post as the road's conditions and threats had become impossible. The Knights arrived later, after the nautiloid crashed, and it is true that they were defending travellers while they hunted for Karlach.
I am under the impression the paladins summoned the gnolls considering the book I found on the balcony desk in the leader's room detailing how to create them...
