Lol after askin some mates, heres a possible reason...
Using ultra pure water causes this. No impurities causes the water to freeze at exactly the same rate. But the problem is, when you put it into anything, contamination is added (through the air, or the container). This contamination collects and causes that part of the water to take longer to freeze.
The surrounding water freezes first, leaving a "hole" in the middle of unfrozen water. This forces the water up... eventually forming a spike.
It sounds like a decent explination... but im still not 100% convinced. Meh.
But this isnt a Funny Ice Thread... lol