One thing you didn't mention - probably because you don't know that - is that your comments in the discussion thread you linked run contrary to a position that's very firmly established on LW: that it's only rational to donate to one charity, the one you judged to offer the maximum utility. For example, this post makes this point very forcefully, but it also runs through several of Eliezer's posts in the sequences. Your arguments in the linked thread can be seen as annoying rehashing of arguments amply covered and refuted in those posts, and that could cause people to downvote you in greater numbers than you would guess based on the comments' content.

However. Having said that, the loss in karma is very heavy, and seems to affect comments of yours that have nothing to do with that thread, and don't really deserve negative karma. Assuming your numbers are correct, I see three possibilities:

1. You're being karmassasinated by random LW users irked by your comments in the linked thread.
2. You're being karmassasinated by a group of LW users working in concert for whatever reason ("hey guys, he sucks - let's all go and downvote his recent comments")
3. You're being karmassasinated by a sockpuppet network.

I find myself thinking that 3 is the most likely explanation given the evidence (that surprises me). However, I don't rate it much higher than 1.