I suspect it's an automatic function when a comment reaches a certain number of downvotes. In this case it may also highlight an issue with not showing points-

I suspect it's an automatic function when a comment reaches a certain number of downvotes. In this case it may also highlight an issue with not showing points- many people may have thought it warranted downvoting, but some may have not clicked had they been aware it was already showing the max of -4.? I know I get the flipside often- comments that may warrant a few upvotes get 20 because we've lost the 'feels about right' element of scores. Of course, I can't fault the converse 'vote on its merits' argument.