Gaps fill because traders and charters believe they likely will...

Gaps fill because traders and charters believe they likely will be filled.
They believe it is likely because history regularly confirms it to be true.
This belief / sentiment can then be sufficient to drive market action and cause the gap to follow it's destiny and fill if the right conditions that influence stock holders actions also support that.
Predicting those conditions correctly on a regular basis is the holy grail of a trader.

But manipulation, general market sentiment and company news are all still in play to move prices while all the technical nuances of a liquid market, that analysts attempt to predict, just happen to occur time and time again, almost, but not quite, predictably.