Power equals Voltage times current.
Voltage equals current times resistance.
Therefore power equals current squared times resistance or voltage squared divided by resistance.
The servo amplifier varies the current to the motor by means of pulse width modulation (PWM). That is it switches the current on and off when it is near the commanded position (small error signal). If it is at zero error signal it turns off. If it has a large error signal it will turn full on for the duration of the PWM period. Analog servos have a PWM period of about 20 msec and digitals have a PWM period of about 3 msec. Therefore digitals send more but shorter pulses for a given error signal.
If the servo is stalled, that is it dosn't have enough power to zero the error signal, the amplifier is sending pulses to a motor that isn't moving and so the load is the resistance of the motor.
Let's say we have stalled the flap servo and the amp has the PWM at 100%. The power into the servo is the voltage squared, say 56.25, divided by the motor resistance say one Ohm so the power is 56.25 Watts. If we drop the voltage the power goes down.
To be fair there is a small range where the PWM is not 100% where dropping the voltage might increase the PWM duty cycle to increase the current but I doubt if that is what is burning out the servos.
Hope this helps.
Jim O