Rorschach is not conscious, which means that it does not spend energy on daydreaming, on emotion, on opinions, or on any part of what we would experience as consciously processing our experiences. This means that to a species like Rorschach, a large part - in fact, the vast majority - of communication between humans is concerning activities that it has no purpose for, and cannot comprehend the need for.
Rorschach only spends energy on things that are directly related to its continued survival. Human activities such as entertainment are therefore considered a complete waste of energy. Even the structure of human language is wasteful - I seem to remember there being a line describing how Rorschach units "time-share" each other's brains. What this means is that Rorschach units don't "speak"; they simply transmit their thoughts in whatever form is more efficient (if I remember correctly, the trapped Rorschach entities communicated by transmitting vibrations through the hull) and each unit continues thinking the recieved thought where the original left off; a true hive-mind, essentially one brain spread across many bodies. Imagine that's how you communicate, then look at the way humans do it: by inventing words with certain meanings, choosing which ones you want to use to express an idea, and then representing those thoughts by a means of sound that includes tonality and stress patterns and tempo, which are all there to indicate social, emotional, and contextual cues... It's tremendously inefficient, from Rorschach's point of view.
When Rorschach encounters this communication, it has to analyse it and work out what it is. Doing so requires understanding all the things I just mentioned, and when it does that, it sees that so much of it is (from Rorschach's perspective) completely unneeded and wasteful. This means that the communication has made Rorschach spend energy on doing something that was ultimately not in any way useful in ensuring its continued survival. Rorschach does not have an unlimited energy budget, which means that by spending some of its energy on understanding human communication, Rorschach is now less able to pursue the activities that it needs to in order to survive.
This means that there is an intelligent creature out there that has done something that has reduced Rorschach's ability to survive. From Rorschach's single-minded perspective, this counts as an attack. The creature who is doing this must be stopped, so that Rorschach no longer has to spend energy on dealing with it, and can instead focus entirely on making its environment safer for Rorschach to live in and ensuring the continuation of its species.
I've tried to phrase this in a way that doesn't imply that Rorschach has opinions or conscious thought; in essence, Rorschach operates solely by what humans would call instinct, reflex, and gut reaction, except vastly more complex; much more direct cause-and-effect and much less reasoning and rationalisation. That's part of what makes Rorschach so scary: it cannot be reasoned with, because it lacks the capability to even think the sort of thoughts that would be required for it to engage in negotiation of the human kind. It cannot be reassured or intimidated, because it is incapable of experiencing fear or concern. The only "negotiation" it would understand is based on actions; humans would have to act in a way that led Rorschach to decide that they are not a threat. Given that this would involve ensuring that Rorschach no longer has to expend any energy in thinking about or reacting to human activities, this is very unlikely.