Ignorance, that's why we're surprised when Buddhists are Violent
If you're a Buddhist, ignorance is no surprise. It is the root of our suffering, the first rung in the wheel of samsaric Dependent Origination. And it is the thing to overcome if one wants to be awakened, the ultimate goal of Buddhism.

If you're an educator, this is also no surprise. We all come into the world with received prejudices and opinions, blinders here and there. Buddhism in the West has the benefit of a very positive public image. Even today, after some scandal, after the ongoing Burma/Rohingya crisis, after the bloody civil war in Sri Lanka (very few non-Buddhist Americans even know about this), and so on. 

The current public image of Buddhism is still rosy.

That is why we are surprised when Buddhists are violent.

Nothing more, nothing less. 

Having taught courses in World Religions, I happily put forth Islam as a good example of the opposite. Westerners have a fairly widely negative impression of Islam. This is due to global political circumstances in recent decades more than anything else. The religion itself is not inherently violent any more than Buddhism is inherently peaceful. Both have great teachings on peace, generosity, selflessness, community, and so on. Both offer occasional justifications for violence. 

In Buddhism, the first step on the 8-fold path is 'Right View' - often better translated as appropriate or balanced view. If we bask too much in the rosy glow of Western perceptions of Buddhism, we don't have right view. This rosy glow is lovely, and thus an easy object of clinging for Buddhists. However, this clinging, like all, is a source of harm if it leads us to fail to see violence and to take responsibility where we can to end it.