I just figured this out last night after thinking about it for awhile because mine was doing the same thing and really bothering me because when the bed hits the extruder nozzle the motor makes an ugly grinding sound which can't be good.

The problem is that the bed adjusted height from the factory is too high for the Z axis limit switch. This switch looks like a tiny lever on the back wall of the printer housing and is supposed to stop upward travel of the bed before it crashes into the nozzle but is not adjusted to cut off in time. There is a screw to adjust the trigger point for the switch but instead of messing with that I just tightened the three hand nuts on the underside of the bed. These are the ones you use to level the bed. I noticed that the bed springs were pretty loose because the hand nuts were close to the end of the screws. So I tightened them up as much as possible. This compresses the bed springs by bringing the bed further down onto the springs. This gives the added benefit of making the be less springy so it doesn't get out of level when you're prying a print off of it. Also, the bed is now about 3/16" / 5mm lower which is just enough to trigger the Z axis up limit switch in time to not crash into the nozzle. Problem solved!