When the water loses energy, the chemical forces (hydrogen bonds in this case) become strong enough to keep the molecules from bumping all around. This forces them to settle into a crystal lattice where each O atom has two H atoms from other molecules next to it. This takes up more space than if the molecules were more randomly scattered, as in liquid state. I found some good pictures here: http://www.iapws.org/faq1/freeze.htm
So, to answer your question, it is the hydrogen bonding that causes the expansion.
