Three reasons in my mind. First, upon gap-up, there is no natural support or resistances...no places where the stock has traded to serve as bottoms for the stock...so there’s nothing to keep it from falling back. Second, there’s a bit of self-fulfilling prophesy to it...it’s like VWAP or a support...if enough expect a bounce off of it, it will...with a gap, we expect them to close...so we don’t buy until they do.
Third, and this is the most questionable but I believe it, is that MMs like to close them. Some of that is reinforced by my second point...but for the most part I believe it’s part of the trading algorithm...their job is to make market, especially if the stock becomes illiquid...and so they’ll close those gaps as a logical target. I find that they usually happen after a bear raid...might as well close the gap when you trip all of the stupid peoples’ stops.
These are for common gaps. Runaway gaps might not cover...or they won’t until and old gap comes back in range because of falling PPS. IMO.