Why are there spikes growing out of my ice tray?
They're often called ice spikes. Ten years ago these things were pretty mysterious, but now Wikipedia even has a video of them growing out of ice cube trays in a freezer.

Basically, the ice surface freezes first, which slightly pressurizes the water underneath. That water breaks through the crust, and continues to flow, and freeze to equalize pressure within the cooling cube. Temperature conditions have to be just right for it to happen, so you don't see them all the time.