I hadn't realised what the KBIC-120 is. It is designed for driving DC motors from and AC voltage that is about the same magnitude as the DC motor. The KBIC-120 doesn't actually reduce the voltage, it just turns it on for some of the time and off for others, at twice the mains frequency.
The output waveform at 12 V will be far from a steady 12 V. When running a 90 V motor, the inductance of the motor winding will keep the current reasonably constant as the supply turns on and off. The supply is turning on and off 120 times a second, which is faster than you can see or your meter can respond to, but it is slow for your motor.
A 12 V motor has far less inductance so the current will be varying a lot more, and probably reducing to near zero at times. This means that the peak current will be a lot higher than it is designed for, so the heating of the windings is too much.
The heating is proportional to the square of the current, so twice the current gives 4 times the heating. Twice the current for half the time, which gives the same motor torque, gives twice the heating.
Also your 12 V motor is not designed for the high voltage peaks that the KBIC-120 gives out. That may be damaging the brushes.
As others have said, get a 12 V supply and a float switch. If you need to control the speed, get a speed controller for low voltage DC motors. They will keep the peak voltage much lower, and they will turn the supply on and off much faster, maybe 20,000 times a second, so the current in the motor won't change so much.