Transportation is one of the key industries of US economy, while paper index traces performance of companies involved in packaging of all the goods shipped.
General understanding is that if fewer goods are being packed and transported, fewer goods are being bought, which in turn means a slowdown in US economy (measured by GDP, which is approximately 70% retail sales).
Thus if stocks of US transportation and paper companies decline in advance of broader market indices, it could well signal a slowdown in US economy not yet accounted for by the general investing public.