Just experimented. It was because I accidentally clicked "paste" over some text, instead of copying it. On this site, at least, you can't retrieve text when that happens
Just throwing this out there, in case it might help people in the future.
Since the site has an (awesome!) recurrent auto-save feature, maybe in those kind of situations the best thing to do would be to immediately close the tab or window - before it can auto-save again - and then reopen it, restoring the post to the last draft saved.
A lot of my writings are code heavy, and because I'm usually quite specific, I don't write with the rich-text editor. So I usually don't have the benefit of auto-save :(
For a while, I did that as a fail-safe. Over time, I just tended to avoid it because it doing the switch could (sometimes heavily) screw with the formatting. But it was damned better option then losing it all