Lots of reasons. Statistical fluctuation. Equipment malfunction. Misunderstanding of setup. Underestimation of systematic errors. Improper data analysis. Someti

Lots of reasons. Statistical fluctuation. Equipment malfunction. Misunderstanding of setup. Underestimation of systematic errors. Improper data analysis. Sometimes, new effects that were previously unknown but don't involve new physics. I once asked a senior physicist what to think about a 4 sigma anomaly that looked like it couldn't just be a systematic effect. He instantly replied "then it's probably two systematic effects pointing in the same direction".