Ice spikes form as a result of the way water freezes, from the outside in.
As the ‘skin’ of ice thickens, if a weakness or hole appears in the ice skin, the liquid water gets squeezed up through the hole.
The water that is squeezed out then forms the beginning of the spike and the process continues until the tip of the spike freezes.
For an ice spike to form, the air conditions need to be just right. It can’t be too cold or the water freezes too quickly, and a little breeze (but not too windy) helps the process.