dogs operate almost entirely on smell, and they can pick up things with their noses that you can only contemplate in the abstract. it's not reacting to the way it looks...
but, put your face up against a fresh, unpeeled pineapple the next time you get a chance. you'll only get a fraction of what a dog will get. but it's enough to demonstrate the point. it has a very territorial smell.
i mean, if you have a better idea i'd like to hear it, but just keep in mind that it's going to be smell-related rather than sight-related. i think we all know this, but i think we easily forget it - because we're so vision-centric, ourselves.
you'll notice that when she puts the pineapple down, the dog smells it from a distance and instantly reacts. that's what's going on...
now, it doesn't have to think it's a dog, exactly, to get that kind of confused reaction. i'm sure the chemical reaction happening in the dog's brain is fairly specific in it's "that's a dog" reaction. i don't know if the glandy smells coming off a pineapple are chemically close enough to trick the dog's brain in that respect (and if it's just a few bonds off, that confusion is entirely plausible - it's about the geometry of the molecules sticking together). but, given that pineapples have such a territorial almost urine-like smell, i'm fairly convinced that it's interpreting it as some kind of living thing, and is just unaware as to how it should react.
put another way, i think the idea that this is a territorial reaction is correct. that pineapple's territorial smell is invading it's turf.
