Question: What kinds of things might bring on a depressive episode after I have been stable for a while?
Answer: Depressive episodes are more likely during periods of stress. They're more likely after you have stopped treatment-- especially medications. They're more likely when you have a medical illness or have changed medications for medical illness. They're more likely if you take alcohol or substances which may affect mood, such as stimulants.
Also diet, regular exercise, and a regular sleep pattern could protect against depression, and changes in diet, changes in sleep pattern, and changes in exercise can precipitate a depression.