Through what mechanism does the voltage rise when brushless motor is decelerated
I assume this is a 4-quadrant motor controller. 

You are pumping energy (angular kinetic energy stored in the motor and whatever is attached to the shaft \$E = \frac{I \omega^2}{2}\$ where I is the moment of inertia and E is energy)- back into the supply rails. 

One solution if there is insufficient storage capacity on the rails (eg. capacitors rather than something like a large battery) is to dump the energy into braking resistors.