It's very reminiscent of the property market following big bang, when (in London at least) prices dropped 20 - 25% leaving many folks in negative equity. Which wouldn't have mattered but that people were losing jobs too and unable to pay the mortgage. I knew several people unable to sell (as the mortgage exceeded the value) and some moved back with their parents and became unwilling landlords, some went bankrupt, and at least one I knew did a runner to Italy. A surveyor friend had a job assessing repossessed properties for any work before being (re)sold: he said it was his busiest work time, ever. This was 1988 - 1994.
Property prices have risen insanely (at least in London, where I live) for a variety of reasons, but to me, from the experience of nearly getting burnt in the early 1990s, two stand out: first, interest rates are absurdly low and have been for ages (remember 1993 when Base Rate was 15%, albeit only for a day?); secondly, at some point the old salary multiplier restriction was replaced by 'affordability'. (It used to be the case that you couldn't borrow more than something like 3x one salary and 1x the second, or 2.25x joint salaries.)
Put shortly, it is easy credit which has driven price rises. Other factors like the advent of BTL in 1997 probably didn't help, although the second most significant factor after easy credit is probably the new build shortage.
On interest rates, when we were looking (and we originally got together solely to afford somewhere to buy, but luckily it blossomed) we would always check if we could afford the mortgage, if rates were 3 or 4 points higher. I vividly recall the building society person's perplexity - 'why would you do this?'
It is however right that negative equity only matters if you have to sell. In the 1990s the lenders took big hits because of foreclosing, and selling at a 'loss', and then being unable to recover the balance from the hapless borrower. I expect they have learned from the experience, but as time passes fewer and fewer will remember 1988-1994.