It's basically the same as humans, they include higher predatory associations and less logical and word associations. The neural circuits study all the images received from the eye and compare new objects to previously learnt categories: Bird/Log/Horn/flailing upside down limb/face/car/rhino... You can feel your own neural networks at work when you focus on an unidentified detail (bird/branch/rock) and class them into right and wrong results. Moving objects occupy a moving class of their own, i.e. in your garden, they are a potential threat or a prey.
When the truck is rolling along normally, the cat has extensively studied this kind of object already and has made these associations: Glass windows he can't get into, faces inside glasses, rubber tires, slight smell of humans, major smell of petrol and artificial stuff, indestructible, square and round bits, sees it every day from all angles and has interacted with trucks plenty of times. He rarely gets any food from safari trucks, they all look a bit the same. A truck occupy the same prey opportunity as birds in trees which can just fly away.
The brain behaves differently towards known objects and movements and new, unknown animate objects, the unknown animate things result in: the capture of attention.
When there is an arm waving out of the window, the cat's brain is receiving an unidentified living thing attached to the truck, perhaps some kind of living meat! A living thing, a prey. Is it a leg of lamb? is it inside or behind the box? Can he get into this truck? Is it a bird? The cat will be interested in the unidentified, new kind of living thing until he has figured it out.
visual cortex of monkeys and cats.
Activation of the hypothalamic feeding centre upon visual prey detection