Why Is Sunni-Shiite Conflict Likelier this Month?

Shiites have begun] the mourning rituals of the month of Muharram in Karbala. They commemorate the martyrdom of Imam Husayn, the grandson of the Prophet Muhammad, at the hands of the Umayyad Caliph Yazid in 680 CE (AD). This is an emotionally fraught season, which in some countries has been attended by some Sunni-Shiite violence (Yazid ruled before classical Sunnism took form, but many Shiites see him as a Sunni oppressor of their holy Imam or religious leader).