How did I break a spindle

[Hipster]

i think you nailed it. good chance this is what happened. lesson learned. is it ok to use the races that come with a new rotor? because if not i got more work to do.

I don't. I knock whatever unknown race out and use bearings and races of the same brand. Usually Timken stuff if I can get it.

Many get lucky and get away with it but for reasons you have experienced I don't like to mis-match and guess on whether the bearing is seating properly in the race by mixing brands. It's a known issue with some imported bearings. A new bearing ALWAYS gets a new matching race. It doesn't matter if it's a wheel bearing, a differential carrier bearing , or on your lawnmower. Unless, of course, it's a sealed bearing unit.

When it comes to brakes , suspension, or steering I don't like to take chances with what may be mismatched or poor quality parts. Count your blessings. This could have had a violent ending.