What causes rotor damage? I was told they are not smooth and need to machined.?

There are several terms that you need to become familiar with. First is variations from parallel= this is the measurement of the rotor at different locations around the 360 degree cirference. allowable is .004 and larger variations will cause a pulsing pedal when the brakes are applied especially from a high speed. Next is lateral run out= this is the readings from the outside edge and moving toward the center, again .004 is the max allowable. This allows the new pad to make full contact with the rotor to give positive braking. Last is a warped rotor which is checked with a dial indicator= observed while the rotor is mounted in place and the dial indicator will show the difference in thickness when the rotor is turned while observing the dial indicator. The rotor can be turned if sufficient material remains, one problem is a shop in a hurry and miss mounts the rotor and cuts is out of round, when mounted and you try to stop, it will feel like warped rotors, even though they have been turned. Calipers seldom go bad but when they do, it will usually cause one side to wear prematurely. The one that wore out too soon is the good one, the one that did not wear was not working. When this happens, it is best to do both to be safe. Most common weakness in the brake system is the slides that will bind up due to lack of lubrication. I suspect this was the cause of your problem.