why does water expand on freezing?

The structure of ice is shown below.
https://c1.staticflickr.com/1/696/32186695436_dafe8cc6b6_o.png

In ice (at 0°C or below), each O atom is surrounded tetrahedrally by four H atoms, two by covalent bonds and two by hydrogen bonds. The structure is rather “open”. When ice melts at 0°C, water keeps only part of the “open” structure. Therefore, ice has a greater volume than an equal mass of water at 0°C. In other words, ice expands on freezing.

Hydrogen bonding causes water to expand upon freezing. The H-bonds cause the molecules to align into a structure that occupies more space than the liquid.

Liquid H2O does not do this because the kinetic energy of the water molecules constantly breaks any H-bonds that form. As the kinetic energy (temperature) of the water decreases, the water becomes more dense, down to about 4 deg. C.

Below 4 deg. C, the H-bond alignment of molecules outweighs the kinetic energy and the density of water begins decreasing.

From Wikipedia:
A simple but environmentally important and unique property of water is that its common solid form, ice, floats on the liquid. This solid phase is less dense than liquid water, due to the geometry of the strong hydrogen bonds which are formed only at lower temperatures. For almost all other substances and for all other 11 uncommon phases of water ice except ice-XI, the solid form is more dense than the liquid form. Fresh water is most dense at 4°C, and will sink by convection as it cools to that temperature, and if it becomes colder it will rise instead. This reversal will cause deep water to remain warmer than shallower freezing water, so that ice in a body of water will form first at the surface and progress downward, while the majority of the water underneath will hold a constant 4°C. This effectively insulates a lake floor from the cold.