What are the bottom group of diodes for on BLDC motor control circuits?
Well, you don't seem to have the same problem with the upper diodes. After all, why would they conduct? For the same reason that the upper diodes conduct, the lower diodes are there for the return current path back to the motor and, the usual reason is when the transistors are off and the motor is freewheeling and generating voltage. Both upper and lower diodes protect the transistors from reverse current and dump the energy into the battery and bus capacitor.