Apparently the TSA has new X Ray machines in some airports and the procedure for going through the line has changed. This just happened to me in Seattle.
Normally, there is a pile of plastic trays and you just offload everything you own into one or two trays and then reassemble afterwards. However in this case, when you emptied your pockets, you weren't supposed to put the stuff in a bin but INSIDE the carryon they are X Raying. Which means all your stuff will be loose and clattering about in your carryon.
Yet another reason I always have a few ziploc bags in my carryon to keep my loose stuff from getting loose. I believe they claim this helps speed up the process and increases safety, because we know that is Job 1 for them.