Circuit design recommendation to prevent a failing regulator
Some things to think about: 


The H-bridge was tried with Vb and micro on a lab supply, but without 5V switcher, right? 
Has the source impedance (of Vb) been compared to load impedance of motor drive and switcher combo? Source impedance needs to be much lower than load impedance. If not, supply could oscillate, especially if kicked by a motor hard start.
Are all the PWM signals generated by timers in the micro, so that they maintain proper phasing and timing (i.e., not bit banged in code)? 
Were all the PWM inputs to MC33883 correct before giving enable? No tri-stated output,or highs when should be low. 
Are you sure the ADC readings from pot are OK? Noise here could cause trouble. 
Is motor drive frequency compatible with switcher filter, both input and output? I.e., not on top of a filter resonance.