Acids are mixed with water to dilute them. The dilution process of acids is exothermic in nature.
If water is added to the acid the heat released can evaporate a part of the water that is being added and cause an explosion which could be very dangerous.
To avoid this, dilution is done by taking water in a container and adding acid to it slowly. The heat that is released is dissipated throughout the volume of water and this keeps the temperature within a reasonable range.
When strong acids are mixed with water, a large amount of heat is releasing. If you'll add water in acid, the concentration of acid solution becomes extremely high and the solution could furiously boil, spreading dangerously acid all over.
When acid is added to water, the concentration of the acid solution is washy and the amount of heat released is small, therefore the solution cannot vaporize, splashing acid out, as previously indicated.
That is why it is forbidden to add water in acid!