Update: на Реддите мой лед раскусили в течение 5 минут:
"I know this one!
Ice cubes tend to freeze from the outside in. This means that you can end up with pressurised water inside right on the verge of freezing which often forms the pressure lines inside ice cubes
Except, sometimes if there is a small hole, the water shoots out of it and freezes on the outside instantly.
The water on the inside of the spike freezes shortly after.
Fun facts: The spikes almost always freeze at a 60 degree angle and are also normally triangular."