This is because of lengthening of the ST segment. Lengthening of the ST segment in turn represents prolongation of Phase 2 of the action potential in the working myocardium. This happens as the calcium gradient driving calcium influx into cardiac myocytes in the plateau phase is reduced in hypocalcemia. This also explains why QT interval is shortened in hypercalcemia.
Reference: Basil M. RuDusky. ECG Abnormalities Associated With Hypocalcemia.
http://chestjournal.chestpubs.org/content/119/2/668.2.full [accessed 13 Jan 2011]