It seems the High Priest understood that Yom Kippur was on Sunday, let’s say, and the Qumranites were on a Monday, so he came down there on their day, perhaps with troops… he wanted them to break their observance.

Source: https://www.timesofisrael.com/first-recorded-evidence-of-yom-kippur-depicts-day-of-affliction-as-sectarian-struggle/