Why has my garbage disposal started tripping the breaker every time?
You probably have one of two possible problems.

1) The disposer may be jammed. Check this by making sure the switch is OFF and the breaker is OFF. Insert the proper hex wrench into the bottom hex socket (center bottom of unit) and rotate it left and right to be sure it turns free and easy.  If jammed, it may take a bit of forceful back and forth action to clear the jam.  Look down inside the disposer with a flashlight to assure there is no obstructions.  If all is clear, turn the breaker back on and try the unit.

2) If after doing step one and all is clear but unit still kicks the breaker, the disposer is probably faulty and may need to be replaced.