Those kinds of mag brakes, like on Millenium Force, physically move the magnets themselves to allow the train to move. The system that moves the brakes was most likely at fault, NOT the magnets themselves. The brakes on Beast and Drop Zone are stationary, which means they are meant to slow the train, not stop it. Therefore, they never move, therefore, they never fail.
End of discussion, the brakes did not fail, rather the system that moves them failed.
*Disclaimer: I am not 100% sure this is what happened, hell, I have no clue if I am anywhere right, but SCIENCE shows that magnets to not turn off, and from what Interpreter said, I came up with this explanation
EDIT: LOL, internet battle