Our DC Motor gives high Current at relatively low load. Why?
That is some serious current.
I would suggest checking your connections are all good, clean and tight.
Take resistance readings across the joints or through the run of the cable, even 1 Ohme across a joint would cause problems at this current.
Look for HOT joints.
Have the cables been changed?
Maybe they have melted or damaged.

Your statement "A DC motor can easily PULL load upto 150 Tons at 750amps" is not true, it depends on the motor, a 6V model railway motor wont do 750 amps.