the current through the motor largely depends on the back EMF generated by the motor as it spins, at low speeds the back EMF is not much and hence large currents can flow, at higher speeds the back EMF increases which limits the current and hence RPM if your timing is fixed. These effects far outweigh any small effects you get from winding or magnet heating. Effectively, the dynamic timing advance ESC's keep the phase of the applied voltage ahead of the back EMF (making it appear weaker) so the motor can draw more current, generates more torque and keeps on revving.
Assuming your battery didn't dump and your ESC didn't start trying to apply 50deg timing at low rpms, only something trying to stall the motor could draw such a large current to pull the battery down that much. Could be anything really, bearings, debris inside the motor, weird drivetrain loading, motor shifted, etc.