The decline in Builders FirstSource shares is directly attributed to the market's reaction to their Q1 earnings report. Typically, a negative stock price movement following an earnings announcement suggests that the results did not meet market expectations. This can be due to a variety of factors such as missed revenue or profit forecasts, disappointing guidance for future quarters, or broader market conditions. Without specific details on the earnings results, the assumption is based on the typical market behavior observed when companies report earnings that fail to meet or exceed investor expectations.