I mean, do people downvote comments that they would have otherwise not voted on if they think the comment has too many upvotes? If not, why do they decline to upvote when they otherwise would have upvoted?

I often (although) not always will upvote a comment simply if it deserves it. I only very rarely downvote or don't vote a comment if I think it is too high but should be positive. Declining to upvote a too high comment is something I do much more frequently than downvoting a too high comment. This is a passive rather than active decision. In general declining to upvote creates less negative emotional feelings in me than actively downvoting something which is too high.

I do sometimes upvote comments that have been downvoted if I think they've simply been downvoted way too much. That seems for me at least to be the most common form of corrective voting.

I have no idea how representative my behavior is of the general LWian.