Hypocalcemia causes weak cardiac contractions because:

Hypocalcemia is the medical condition with low calcium levels in the blood serum.
Low calcium levels blood can weaken the bones, create kidney stones, and interfere with the way your heart and brain works.
The effects of Hypocalcemia on the heart are weak contractions, arrhythmias, and low BP., weak thready pulse. 
Hypocalcemia causes weak cardiac contractions because it increases the resting membrane potential and the  excitability of the cardiac muscle.