Typically this happens when two single coil Fender style pickups are combined for example:

1. Each pickup is wound in an opposite direction. One bobbin is wound clockwise and the other is wound counter-clockwise.
2. Both bobbins are wound with the same magnetic polarity. Either both “South” or both “North”.

When the two pickups are combined in parallel the pickups will be electrically out of phase because of the opposite winding on each pickup.