Acids react with water vigorously, to produce large amounts heat .
We should always add  acid to water in drops, while doing so, the heat generated is comparatively lesser and the present water is easily able to neutralize that heat.
However, if we add water to acid, a huge amount of heat is released at once, also the water can't  neutralize it anymore as its present in lesser amounts now.
This may cause the acid to splash out form the container, resulting in serious harms.