The bull flag starts with a strong price spike higher that often catches traders who are short the stock off-guard. The many market scanners do their work and identify the long opportunity, so momentum traders then get in long to help feed the buying frenzy and push the price higher. Eventually, the initial price spike runs out of steam and the price action starts to form an orderly pullback where the highs and lows are close to parallel with each other. Drawing the diagonal support and resistance lines forms a sort of tilted rectangle.
