Q:

Why does hypocalcaemia cause increased muscle contraction?

Calcium is needed for  muscle contraction, so how does hypocalcaemia cause increased sustained contraction?

A:

Consider the pathophysiology of tetany, which is the involuntary contraction of muscles caused by hypocalcemia.  
From the tetany wikipedia page:

Low ionized calcium levels in the extracellular fluid increase the permeability of neuronal membranes to sodium ion, causing a progressive depolarization, which increases the possibility of action potentials. This occurs because calcium ions interact with the exterior surface of sodium channels in the plasma membrane of nerve cells and hypocalcemia effectively increases resting potential (rendering the cells more excitable) since less positive charge is present extracellularly. When calcium ions are absent the voltage level required to open voltage gated sodium channels is significantly altered (less excitation is required).

So, when Ca2+ levels are low, action potentials are spontaneously generated, leading to muscle contraction. 

