It appears to be part of the spider mutation, just like the spider pattern. After many generations of outbreeding it still follows the spider mutation. Occasionally you will hear of a non spider wobbling but presumably that is some other random cause so I wouldn't expect any of the non spider siblings to wobble but I would expect all the spider offspring to at least have the tendency. Why some spiders do it more than others I don’t know but would be very interesting to find out as it might help minimize the effects.