How does lightning break a tree branch when it hits it?
Wet wood is a poor conductor of electricity. In the cloud to ground circuit, the tree is a resistor. The large resistance dissipates part of the strike’s energy as heat. The water in the wood heats extremely rapidly to far beyond the boiling point, exploding to steam and shattering the wood.
This is actually more of a physics question. Basically, the lightning heats the water contained in that trees wood, which expands, thus breaking the wood.