why does liquid nitrogen cause a pop bottle to explode?

Answer 1

The question needs to be clearer. I assume you pour some liquid nitrogen into a pop bottle, seal it, and it explodes. Two things are happening. First the liquid nitrogen freezes the bottle and makes it brittle like glass. Then the nitrogen turns to gas and quickly expands inside the bottle. At room temperature, a pop bottle would expand like a balloon. At -100 C, the bottle does not expand and eventually cracks under the intense pressure.

Answer 2

Liquid nitrogen boils at a very low temperature and as it does so it turns to nitrogen gas and the volume expands greatly. The pressure generated is higher than the bottle can stand and - BANG. I do not recommend you try this as the razor sharp fragments of glass reach speeds nearing that of a bullet and can maim or kill if they hit you even if you are a long way off.