"If prices fall even more, the banks will call for topping-up (not unlike a margin call)." Will never happen especially in the scenario you describe, as most would not have the funds to top up as you suggest. The banks are primarily interested in their payments being made as long as the people who have taken out the loans do that they will be fine. If they don't make the required payments then ultimately they will be sold up at a loss to both the owner and the lender (even though the owner will still be liable for the outstanding loan amount). In no way is this in the interest of the banks to do this as it would make the scenario you describe much worse than it would have to be - especially if payments were being made as required.
