Comments generally get disabled because people were Saying Mean Things. Next time you see that, look at how many likes/dislikes the video has--odds are it may have a lot of dislikes. Or the owner could just be taking a stand against the terribleness of Youtube comments by not allowing any.
No, I can't see comments on old videos that I'm 90% sure had comments before. Though it seems it's only on a couple channels, so maybe they have retroactively disabled comments on all their videos.
