When a trendline support is broken, it often starts acting as a resistance because market psychology changes at that level. Traders who earlier bought near the support and are now in loss tend to sell when price comes back to the same trendline, creating selling pressure. At the same time, new sellers see this retest as an opportunity to enter short positions, expecting the prior support to hold as resistance. Due to this shift from demand to supply, price usually struggles to move above the broken trendline, confirming it as a new resistance zone.