Because I have owned cats all my life, I believe those cats know exactly what they are doing. They are taunting your dogs, because that's fun for them. The dogs may come close, but I would be really surprised if a cat ever got grabbed. I presently have a cat who is 15 years old and suffering with kidney problems - she has terrorized my 7 month old female Basenji, who has always tried to get her to play or just "get" her, and my B has cat scars to show for it. Finally, in the past couple weeks, the "leave it" command is working - but mainly she has learned to give the cat wide berth. I know Basenjis hunt small animals including cats, but the fence-walkers are sure footed and quick enough to jump the other way if the dogs come close.