According to WebMD, kidney cysts are closed pockets filled with fluid or tissue. They can become infected or start bleeding, in which case they are called hemorrhagic. They can also increase in size and become painful when pressing on the walls of other organs.
Medscape mentions that the risk of renal cysts becomes greater with age, especially in people over 40 years old. Acquired cystic renal disease is common in this age group, and 50 percent of patients with this medical issue develop hemorrhagic cysts. Wikipedia adds that patients with compromised immune systems are at a greater risk and that hemorrhagic cysts may also develop in women's ovaries.