This adds to the earlier decision from Fitch here, which is pretty much the same story. DBRS Morningstar notes that the review with negative implications "reflects the risk of Congress failing to increase or suspend the debt ceiling in a timely manner". Adding that:
"While we still expect Congress to raise the debt ceiling before Treasury runs out of available resources, there is a risk of Congressional inaction as the X-date approaches. DBRS Morningstar would consider any missed payment of interest or principal as a default. In such a scenario, the relevant U.S. Issuer Ratings would be downgraded to “Selective Default.” "
You can check out the full post here.