The quantity of money in the system doesn't stay the same. Speculators buy financial assets such as stock and real estate on credit. This increases valuations and causes more speculators to buy financial assets on credit, or at least stop paying back their other debts as fast so they can hold more assets, to capture asset price gains. When the process slows down eventually the expected asset price gains will be lower than the interest owed on the debt, causing a sell off.

That's the basics of how a credit bubble works.