what's with drooling

i have taken in a coupla ferel cats , now neutered ,same mama, different litters . she brought them to the feed a few times when they were little, to show them where it was , then left them with me . now , question , why , when they are being scratched , do they slobber ? they just drool a big wet spot . when i was a kid , daddy always had cats , 17 , one time , and i don't remember them doing that . sally