The Materials Select Sector SPDR ETF (XLB) experienced a significant -2.65% decline in the after-hours session, moving from a closing price of $50.89 to $49.54. This drop appears to be linked to a broader market downturn rather than a specific negative catalyst within the materials sector. The after-hours session saw weakness in major technology and semiconductor stocks, which likely created a risk-off sentiment among investors, impacting cyclical sectors such as materials.