Why do the stand mixer attachments rub the bottom of the bowl?

You'll likely need to adjust the attachment clearance setting when attachments rub the bottom of the mixing bowl.

To adjust attachment clearance on common KitchenAid mixers, turn the speed control off and unplug the stand mixer. Tilt the motor head up and install the flat beater. To raise the flat beater, use a slot screwdriver to turn the attachment adjustment screw (located at the hinge point) 1/8th of a revolution counterclockwise. Lower the motor head and check the flat beater's clearance. If the beater still touches the bottom of the mixing bowl, continue adjusting the clearance setting 1/8th turn until the flat beater just clears the bottom of the mixing bowl. Plug in the power cord and turn on the mixer to check beater clearance while spinning. If the flat beater strikes the bottom of the mixing bowl while turning, turn off the mixer, unplug the power cord and continue to adjust the screw and check beater clearance until the flat beater no longer strikes the bottom of the mixing bowl while spinning.

If you have a different type of mixer, follow the instructions in your owner's manual to adjust the attachment clearance setting.