Price action is created by the interaction of individuals in the marketplace. Bottoms are often characterized by capitulation, which by definition occurs when some material portion of asset-holders collectively decide to puke out of their positions - based on newsflow, recent action and other factors. Human psychology is similar from person to person, leading to the phenomenon where many such weak hands reach their uncle points at approximately the same time.
Note that the reverse is also true i.e. weak hands will capitulate into a strongly trending asset at approximately the same time, usually after a large move has occurred and fairly close to the turning point. See for example the ARKK NAV over time, most of the money piled in right at the top. This means that when the tide turns, everyone starts seeing red at the same time and to approximately the same extent.