The reasons are
Primary bacterial infections: ca. 3%
crystals (either struvite or oxalate) ca. 20%
Urethral plugs consisting of tissue and struvite and oxalate sands: ca. 20%
So called idiopathic- which means no known reason ca. 55%- the reasons here might be found among unknown viruses, autoimmune problems etc.

All of the latter three reasons can come with a secondary bacterial infection (which leads to pus and blood in urine).

For all these problems, drinking a lot is important. The crystals and urethral plugs may be prevented by the right diet.

If they have an UTD (like Urinary Tract Disease, because it is not always an Infection) there is no absolute way to prevent it although drinking makes sure the bladder is flushed regularly which is very good for all of the causes. They usually get antibiotics against the primary or secondary infections. In the case of Idiopathic, it can go away once the secondary bacteria infection is under control. Sometimes prednisone helps to keep the primary inflammation down (although it does not remove the causes).