When a material shrinks, it can create immense internal stresses equivalent to what it would experience at extreme pressures. Compare to say a Prince Rupert's drop.

I have a theory of superconductivity that also explains that, unfortunately this comment box is too small for me to explain it.

That's very interesting! I remember a while ago I read about a room temperature superconductor... but it required enormous pressures.

This is how I explained LK-99 to a friend a couple days ago :P