People just indiscriminately sell whenever they get a good bounce, or even when they don't. Short sellers ride their backs as the bids on stocks keep dropping. Then when the bounces come, they're hellacious because the sellers have been temporarily exhausted and shorters have to cover, fueling trader buying. Of course, the market eventually gets to a point where all the panicking sellers just capitulate and people begin to look at what money they have available and then find that certain stocks are at attractive levels for once. Then the buy, and the process becomes anew.
Bear markets tend to last much shorter than bull markets.
At least, that's the way it's been for most of the twentieth century. Boy, I hope not. I watch very experienced traders getting excited about any sort of bounce to the long side. Yet they persist in only playing long, never shorting. It's a mental block for them.