They don't like you claiming systems next to their systems. And they don't like your colonies next to systems that they have claimed. They won't IIRC claim a system until they colonize it.
It's a defense mechanism given to the AI because they aren't smart enough to analyise the situation and be able to tell if you are a legitimate threat. So they are told to assume you are a threat as soon as you get close.
It's frankly why PBW is so popular.
Geo