Generally, at least some of the income from an asset is used to repay financial obligations associated with the asset’s purchase. With speculative bubbles, future capital gains - rather than future income - are often the main source of expected profit.
If the bubble bursts, and such capital gains aren’t forthcoming, people have to look elsewhere for the money to service and repay their debts. Debt financing is an extremely useful feature of the economy. It facilitates the reallocation of resources in the economy towards the most profitable activities. Nevertheless, heightened debt can seriously backfire when bubbles burst.