It happens because investors are pessimistic about where the economy is headed, and this causes the declines to sustain themselves over a long period of time. Investors sell their shares, anticipating losses, and other investors see more losses in their portfolios so they sell also, and so on.
The bull and the bear have become symbols of the financial markets. A bull market comes in good financial times when stocks and other indicators such as employment rates are up, while a slump is called a bear market.