Diuretics (water pills, caffeine, alcohol) can be a cause. The most popular prescription diuretic (furosemide, aka "Lasix") is what is known in medicine as a "potassium-wasting diuretic" because it will cause you to lose potassium through your urine (as well as reduce sodium levels in your bloodstream).

Another common issue that can cause someone with already low potassium levels to see them go lower is insulin administration. Insulin causes a shift of potassium out of the bloodstream and into the body's cells. 

Potassium needs to be in a pretty tight range in the bloodstream - about a quarter of the range of sodium in your bloodstream. The typical range for potassium in the blood is between 3.5 to 5.0 millimoles/Liter of blood, often written as "3.5 to 5.0 mEq" here in the US.

Below 3.5, you start getting weakness, vomiting, then (as it progresses lower) irregular heartbeats in both number of heartbeats per unit of time and the actual trace you see on the EKG. Above 5.0, things start with tingling, nausea/vomiting, then irregular heartbeats, and if it goes high enough, the heart just stops. Above 6.0 mEq is considered an emergency, and one of the treatments in the ER for too high potassium levels is to give the patient IV insulin with a sugar on an IV drip.