When acid is poured into water, the solution that is created is diluted and produces little heat. If water is poured into acid, the solution created is a very concentrated acid. In this situation the acid produces a large amount of heat, which makes the solution volatile.
When water is added to acid, rather than the other way around, the acid gains strength rather than losing it. This is because the acid releases heat when combined with a base, such as water. As the heat is released, it can cause the mixture to bubble, potentially bubbling so violently that it can come to a boil very quickly. The bubbling acid is extremely concentrated and can be hazardous to both people and the surrounding environment if it splashes out of the container. Adding acid to water still creates heat but the amount of heat created is so minor that it dissipates very quickly and does not pose a threat as the acidity of the solution is reduced by the introduction of the acid into water. When acid is added to water, rather than the reverse, the solution created poses no danger of bubbling over or heating to the point of boiling.