Sudden pressure rising in a pipe of an old plant transporting 33% ammonia solution
Ammonia is more soluble at lower temperatures so the temperature change between the outside of the factory and the inside could raise the pressure inside the pipes.

Gas dissolved in water are metastable systems most of the times. Nucleation points in many cases suffice to promote the release of the gas from the solution (have you ever tried to put some salt in your beer?). It is possible that somewhere in the old pipes there are some areas with higher roughness (e.g. rusty connections) where some nucleation points promote bubble formation and so the ammonia can be released.