A bear market is generally defined as a decline of 20% or more in a major market index like the S&P 500 or the Dow Jones Industrial Average (DJIA). Since 1932, bear markets have occurred on average every 56 months. Those which occur outside of a recession tend to be shorter.
There are actually two types of bear markets: recessionary and non-recessionary. Since 1928, 14 bear markets preceded recessions, while another 11 bear markets since 1928 had nothing to do with recession.
Events which can lead to a bear market include higher interest rates, rising inflation, a struggling economy, military conflict or geopolitical crisis are among the usual suspects.
When the market turns bearish, almost all stocks within it begin to decline, even if individually they're reporting good news and growing earnings.