What would cause the shape of a molecule to be tetrahedral?

A molecule is tetrahedral if the central atom has four bonds and no lone pairs.

Explanation:

A typical example is a molecule of methane (see image).

The electron pairs in the bonds repel the electrons in the other bonds, so they all try to get as far from each other as possible.

They can do this if the bonds point to the corners of a tetrahedron, with bond angles of 109.5°.