Consider changes of coordination centre (A) electronegativity. When decreasing the electronegativity the pyramidal inversion barrier increases (that means it's harder to do the inversion) and therefore H-A-H angles decreases. This is the of possible explanations. You can translate this language of the inversion barrier into the language of molecular orbitals very easily, just consider the molecular orbital diagrams of two possible conformations (with large and small angles).