I’m making an educated guess here…

The initial blast was along the grain of the crystals, the delayed cracks come from the shockwave travelling back after hitting the bottom of the lake and striking the ice at a diffuse range of angles.

Might be those kind of firework that shoot out into the sky and explode, so the initial blast is weaker and didn’t crack the ice, and then the second stronger blast crack it. You can kinda see the second blast in one frame before the ice crack.