The tree splits due to the pressure of the boiled sap inside it, not due to the lightening chopping or physically striking the tree.
This happens in a tree because there is enough resistance to the flow of electricity. When there is enough oomph, like a lightening strike, the electricity is able to overcome the resistance but this creates heat and a lot of it.
There is a lot of moisture in trees aside from the normal sap and this will boil into steam when heated up.
Steam takes up much more space than the liquid it used to be and because it cant escape this will cause the tree to rupture, split, or explode.