subordinate debt rated just above junk BBB+

BBB+ is only 4 ratings above non investment grade.

BHP subordinate debt has fallen from A- to BBB+

In finance, subordinated debt (also known as subordinated loan, subordinated bond, subordinated debenture or junior debt) is debt which ranks after other debts if a company falls into liquidation or bankruptcy


CREDIT RATING EXPLAINED
https://en.wikipedia.org/wiki/Credit_rating


Summary.
This is a red flag that in the event of liquidation that BHP subordinate debt is near worthless and
Non-investment grade.

Outcome
This is an indicator and benchmark that BHP will have to raise capital to install confidence in the company as a long term investment and solvency.

At this point if BHP doesn't raise capital it will go down the path of Santos and the market will be too afraid to touch the shares. This will result in a total exit of institutions.

Time frame.
BHP now have about 7 days to work towards a capital raising.