Van der Waals repulsions
Because severe van der Waals repulsions occur among the tert-butyl groups in compound A at normal bond lengths and angles, the only way for the molecule to relieve these repulsions is to stretch the appropriate bonds and flatten itself by widening the C-C-C angles. The observed structure is a compromise among the Scylla of van der Waals repulsions and the Charybdis of nonoptimal bond lengths and angles.