Why it happens:
You either have an extremely vertical swing plane (like a Ferris wheel) throughout the swing, or you take the club back too far inside and then come "over the top" into a steep position and cut across the ball through impact. Here are some drills to shallow out a steep swing.

MAKE A BIGGER TURN
Curtis Strange's head and upper body moved well behind the ball as he swung the club to the top. To get a feel for how much he turned, grab your driver and address a ball off the inside of your left heel. Now turn to the top with the goal of getting your lead shoulder over your back knee. Go slow in the beginning; I don't want to put you in traction. Getting the lead shoulder well behind the ball promotes a shallower angle on the downswing.