There is a catalyst, at least for what has happened since the FED decision, and remember that there was a sizable reversal on announcement day. I'm surprised that people who either are or want to act like they are professional investors haven't discussed this much, but at the same time the FED announced their no hike decision, they also reduced their target ranges for:
(1) GDP
(2) inflation
(3) Fed Funds rate.
http://www.federalreserve.gov/monetarypolicy/files/fomcprojtabl20150917.pdf
There was initially a move up, but that reversed quickly when traders had time to digest EVERYTHING, and I think this is all short term thinking, but they are thinking what else is the FED not telling us? The changes by the FED weren't massive, but I think there is some suspicion that the FED isn't totally sanguine about the US economy. Yes, the World Bank and the IMF asked them to not hike rates, but there may be more to the issue.
This may be nothing more than some initial selling that triggered the retest, but it is interesting that the recent GDP revision has not changed anything.
As for the commodity collapse, what is not being discussed generally is that in the 2009-2012 (and later in North America for oil) period, there was significant increased capacity added--particularly South America copper mines--at the very time that industrial production was topping. So, the present situation is not JUST a demand issue, and demand hasn't fallen as much as prices would imply, but a supply/demand issue the same as with oil. Yoe saw an example of the supply issue with CAT announcing significant layoffs for mining/drilling equipment jobs last week.