Moody's just reignited concerns about the US debt mountain. Here's what that means for the troubled Treasury-bond market.

• Moody's lowered its outlook for the US's credit rating to negative Friday.
• It flagged political polarization and the government's mountain of debt as economic threats.
• The ratings agency's announcement comes during a period of turmoil for the Treasury-bond market.

Moody's Investors Service dropped a potential bombshell Friday, with Treasury-bond markets still in a state of turmoil.