Sometimes, when I insert a '>', it will fill in automatically the text that I have copy-pasted. I only use Reddit for this one sub, so I don't really have enough experience to know the rhyme and reason to it. My question is, does this look at your clipboard, which could have any assortment of sensitive info, or does it use some other mechanism?
If you have text highlighted on a comment and click on reply it will automatically be quoted for you. You might be seeing that happen.