If just the surface of the water was frozen, then it's possible something (a bird, for example) hit it and disturbed it. Then, exposed slightly farther away from the rest of the ice, this might have melted and later refrozen. A solid "chunk" which landed at the top would have melted more slowly, leaving looser debris to quickly melt, leaving only the pillar. If a small layer of water melted at this time in addition to the debris, it would have been enough to fill in the hole.

This is a spectacular, but typical example of an effect that one sometimes gets when water freezes in a rigid container, such as an ice cube tray or a bird bath. When conditions are just right, the right temperature for the size of container, not too much salt in the water and so on, then first the top freezes over, forming a box of ice full of liquid water. The water continues freezing, and expands in the process.  If a suitable piece of ice on the surface breaks off, or a suitable crack forms, such that the water wells slowly out, the surface of the extruded water freezes over in its turn and breaks in its turn, creating any of a wide range of shapes. On an ice cube you typically get an ice spike, but one gets all sorts of shapes, blades, hollow prisms, solid prisms, rods, mushrooms, you name it. 

What you have here is a very nifty mushroom. Any search program will find you several sites some quite helpful. You could begin with:

http://my.ilstu.edu/~jrcarter/ice/

Good luck,

Jon