Would anybody explain how generally bear market happen? or Stock market bubble burst?

I am fairly noob when it comes to stock market things and I don’t have any significant investment in stocks. Recently I started following stock market more and specially after the spaceX IPO. It seems that spacex is managed to sell 75bn dollars worth of share. Moreover stock market also closed at all time high and there is a demand for tech stock all over the world.

My question if there is so much demand everywhere, how can that demand evaporate in such short amount of time when bubble burst. e.g. One day investor are willing to buy a share of xyz company for 100 dollars but next day they will get hard time to find a buyer for 70 dollars. How does this mechanics works? You can also suggest a good documentary.