Think of it like following a playbook, given market and price...

Think of it like following a playbook, given market and price action is determined by people, or very fast algorithms trained to mimic price action and trends set by real people, traders follow the "gaps will eventually fill", unless there is a catalyst in the form of a macro event or company announcement that changes the playbook in context to momentum in whichever direction, such that noone bothers to "chase" the gap, if the sentiment is negative in the direction of the gap, most traders/algorithms will chase the play, just like if say in a boxing match your opponent is covering his face, you will instinctually go for the low body blow. There can ofc be more to it, but from a trader's perspective this is one explanation why gaps tend to fill.