It seems random but there is a certain logic to it. First, there is an original post. Obviously, the first response immediately follows it. Subsequent replies may either appear above the first response or below it (it's up to the indidvidual posting them, and whether they are responding directly to the original question or elaborating on one of the replies) Often, a secondary topic will be raised by one of the responses, and people will start replying to that (usually below) If this happens more than once, the thread will become very fragmented, and can be hard to follow.