This may overlap with some of the other issues listed, but I think it comes from a slightly different place mentally/emotionally: They're purposefully trying to disengage early rather than getting into a fight about who has the "last word" on the subject, e.g., on some level they may want to respond or even to "win" the exchange, but they're purposefully telling themselves to step away from the computer.
I have consciously done this before. Especially growing up, I was always obsessed with "being right".
I often feel guilty for ignoring other people's comments or questions, and frustrated when other people seem to be ignoring me. If I can't indicate to someone exactly why I'm not answering, or can't receive such an indication myself, I can at least help my future selves and others obtain a better probability distribution over such reasons. To that end, I'm listing all of the reasons I can think of for someone to not respond to a comment/question, to save the effort of regenerating these hypotheses from scratch each time and prevent the possibility of failing to consider the actual reason. Note that these are not meant to be mutually exclusive.
If I missed any reasons (that happen often enough to be worth including in this list), please give them in the comments. See also this related comment.