As housing became more and more expensive to young professionals, some people in this group have worked harder and harder to buy property, even to the point where it no longer seems rational. For example, parents taking a lot of wealth out of their retirement savings or their own homes to assist children in buying. Professionals are working more than otherwise makes sense for their stage in life (young families with two full-time parents). They are committing a large share of their monthly budget, often right until the start of retirement.
They do this because they believe in the importance of owning property - beyond any reasonable narrow economic justification.
Of course, there is an inequality aspect to this - not everyone's parents have capital, not everyone can command a high enough wage.
But crucially, the presence of this group of people arguably turns the bubble into something else. These buyers put a floor on the market. If prices drop even a little, or something else changes to make mortgages slightly more affordable, they rush in and buy the dip. By doing this, they sustain the high prices for everyone else in the market.
This will probably happen now. Higher rates will make current prices unsustainable. As soon as they correct to the point where monthly payments are back to what they were last year, there will be buyers, only too happy to overextend themselves to get out of renting.
It's inaccurate to characterize these people as likely to default. They are actually very good mortgage risks - they have already shown themselves to be very committed to ownership. And the resources which got them into a position to buy mean they will keep on paying short of a disaster. The truly unrealistic borrowers of pre-2008 have never been let back into the market.