One of the many side effects of chemotherapy is hypokalemia.

Chemotherapy can damage healthy cells such as those that line the interior of the digestive tract.1 The result can be a disruption in the delicate fluid balance that these cells maintain. Specifically, absorption of fluid from the gastrointestinal (GI) tract back into the body is decreased, and secretion of fluid and electrolytes in the stool is increased.

The extreme diarrhea that is a side effect of chemo lowers the levels of electrolytes.  While the body tries to regulate is production of both potassium and sodium, if one goes down, the body will decrease production of the other to compensate.

Another potiential reason for hypokalemia is that the radiation that is typically included as a part of chemo will damage the lining of the intestines and makes it harder for the body to absorb the potassium in food.
http://www.chemotherapy.com/side_effects/other_side_effect/diarrhea.html

http://www.health2010.org/gt/view/id-2839