On top of what jtsummerssaid, users with enough karma can 'vouch' for dead comments and get them revived. I've no idea what the threshold is, but I can vouch for those comments if I wanted, you've got to click through to the direct comment page to do it (click the time of the comment, it's a link).

I suspect another user has done exactly that for the comment that was suddenly visible, someone vouched for it.

On the other hand, I wouldn't say that users comments are great, bit conspiracy sounding, insults another user, I wouldn't vouch for him.