Smoked outrunner questions

I was running a 600kv outrunner in my direct drive elst on 4cells. I put a 400kv motor in it and it also ran ok on 4 cell. I ran it on 6cell and thats when things went bad. It was better off the line but after about 75% throttle it would cut out and screech like a stuck rotor on start up. If Even if I tried to accelerate slowly it would still do it. After a few passes up and down the street smoke was pouring out of it. I unplugged the batteries and found the motor very hot. I pulled the motor out and it hardly turns at all now. Both bearings seem good, magnets are still good so what gives?