love detective
there's no love too small
Suddenly subprime mortgages become liabilities that you know will not recoup their value. Interest rates reflect perceived risk, and suddenly the perceived risk was much higher.
Just to clarify (once again), the reason for the increase in payments was not this, but a much more simpler and straight forward technical reason - it was the expiry of the teaser interest rate period that increased the payment so much, as mentioned typically moving the interest rate from something like 7% to 12%.
Once your mortgage has been agreed, the interest rate doesn't go up on it if the bank thinks you are now riskier than it thought when they gave you it (interest rates can go up in general across the board, but that effects all borrowers, and the margin that lenders charge on the loan is set at the commencement of the mortgage - it can't be altered once the mortgage has been agreed - mortgage providers/banks can't target individual borrowers and raise their interest rate on their loans just because they've re-assessed the risk on that loan). If you are now a worse risk than they originally thought, they are stuck with that and it's their problem (or whoever ultimately ends up owning the debt) that they didn't assess the risk properly at the outset. Increased rates for lending, due to a reassessment of risk, can only be applied to new business/new mortgages, not already transacted ones
While it clearly did become apparent that sub prime lending was not going to recoup what it had lent out (once the housing market started to stutter then crash) it wasn't a case of interest rates going up on that lending because of that to reflect increased risk. It was the inability of borrowers to meet the payment at the original normal interest rates (i.e. the rate after the teaser period had expired) that broke the back of the thing
edit: I know that in hindsight looking back at this in the position we are in now, it seems completely absurd that this situation could have ever happened in the first place, but i guess that just shows the inherent absurdities of not just financial capitalism but capitalism in general