What is the main cause behind the fluctuation in real estate prices?

In a perfectly competitive market, the interaction of demand and supply determines the Price for which a property is bought or sold. The price of a property is determined at the level where demand equals supply. However, where there is a shift in demand or supply of housing in the market, Property prices tend to fluctuate.

Generally, the tendency for property demand, supply, prices and returns to fluctuate around their long term trends or averages constitute a Property Cycle.

when supply is low and demand is high, this cause fluctuation in real estate prices

Selfishness acts of the few, also known as Greed.