User blog comment:Crazy sam10/CoDWiki News - Novober 23 2014/@comment-1916935-20141124194029/@comment-1187559-20141124194304

I think they're designed to only pop up after a death to avoid them getting in the way too much. I think some I've seen pop up after getting a challenge, but I think the death related ones are to do with time played, and to stop them popping up semi-randomly when you're in the middle of combat or something.