A trendline support breakdown happens when price decisively falls below an upward-sloping support trendline that was previously holding higher lows, signaling that buying pressure is weakening and sellers are gaining control. This breakdown often indicates a potential trend reversal or at least a corrective phase, as traders who were buying on dips start exiting their positions. If the breakdown is supported by strong volume, it adds credibility to the move and increases the probability of further downside, while the broken support trendline may later act as resistance on pullbacks.