In Yemen there are two large groups at play, the Saudi backed Sunni residing in the North and the Iran backed Houthi Shi'ite. These two groups once shared mosque and had been able to keep tensions from exploding has had just that happen. Three weeks of combat between the two has started out to be regional in nature has turned more and more into a sectarian fight as the Shi'ite claim they are defending their land and against marginalism while the Sunni claim they fight in the better interest of Yemen. The terrorist group Al Qaeda has been operating in the region for months now committing suicide bombings against Houthi mosques and attacks on off duty soldiers. These attacks, along with centuries of various disputes have seen the lines between sectarian and regional blur more as the fighting and outside influences continue.
It's a shame such things happen but it seems more and more Muslims have began causing internal strife within their own territories as seen by the likes of ISIS and Al-Shabab. Though blood feuds between Sunnis and Shi'ite have been happening since the early Islamic period one would think such a thing would be left in the past as most religions have ended the crusade mentality against their own kind. These kinds of conflicts also have the added casualty of the reputation of Islam as a whole being seen more as a terrorist religion and one of violence to many, rather than a religion of peace and discipline that many Western Muslims claim it to be.