I think sometimes the situations our dogs find themselves in dictate their behaviours more than the people that surround them...well at least that's what I tell myself when Ollie acts like a little
. Not so much now but in the past at times it has felt that our relationship counts for nothing - if he switched into guarding mode then all that mattered to him was his ability to defend his possession or his space against whatever threat he perceived - and in his mind, in that mode, we constituted the same threat as any one or any dog else.
I think that, like bizzylizzy is saying, that unusual shapes and silhouettes can provoke more of a reaction - Ollie is thrown more by hoodies and hats and fancy dress. I think dogs can have a bit of an in-built predisposition to react to novelty - some dogs have inherited a more bomb-proof disposition and are pretty unfazed by most things, other dogs are naturally more reactive and respond with fear to novelty.
The back of the car was a space Ollie regularly used to guard but doesn't at all now. It used to be if he hopped in with his lead still on we could not try to unhook him without bloodshed or have any hope of plugging his harness into the seat plug socket. I think this was partly because this was a space that felt inescapable to him and that can be a trigger for dogs, a bit like some dogs have leash reactivity but are fine quickly greeting other dogs off lead. It also probably began to feel very much like his own space, as no-one else sat there. I don't know what changed, I don't think this was an area we really tried training out the behaviour, but I think it probably went away a few weeks after we had him neutered, at the same time a lot of his behaviours improved dramatically without any other real explanation. Is Henry neutered?
The other thing to check, if this is a quite sudden change in behaviour, is that he doesn't have any health conditions or pain that are causing him to act in this way so a vet check might help.