On the coasts, rents & mortgages are no longer connected
so that's one reason why many think we're in a housing bubble.
Also, look at the percents for various types of mortages; in prime bubble areas, the riskier loans are common, e.g. this year SFBA I/O loans are over 50% of new mortages.
Money is being concentrated into fewer and fewer pockets and those pockets need to do something with it. The wealthy are competing with each other to buy homes, driving the price of homes way up.
They must do something with those homes and about all you can do with them is rent them out. This has resulted in a glut of homes for rent with rusulting downward pressure on rent.
The truly wealthy have reduced real instate investments.
However, there is a lot of speculation in real estate, people buying homes based on assuming that the price will go up. That's the only way an I/O loan makes sense. And it's not the wealthy that are taking out all those I/O loans here; my guess is that it's a mix of speculators and people feeling "If I don't buy now I'll never be able to buy, and I'll be able to refinance in five years".
Only time will tell what happens, but it seems a lot like the dot-bomb boom to me.