It starts with "I can assume what this person is about to say". Because you can project how the conversation will go (You're in "good company" - family, friends, hackernews colleagues - you are all used to linguistic shortcuts as well)
The reality of what they say differs from how you imagined it. Maybe it was close, but they participated new information, so now you feel the urge to contribute as well. Add your own perspective.
You are now suddenly in an arms race with the other person to find the "correct perspective".
That's really insightful. I definitely do this! Even when I correctly anticipate their point it's not a good conversational approach - and when I get it wrong, it's a major miscommunication. I think for me it happens when I prioritize information over people. I'll leave it as an exercise to the reader to extrapolate that point across the HN demographic.
The reality of what they say differs from how you imagined it. Maybe it was close, but they participated new information, so now you feel the urge to contribute as well. Add your own perspective.
You are now suddenly in an arms race with the other person to find the "correct perspective".