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

Both pickups are wound in the same direction either clock wise or counter-clock wise and the beginning of each coil is grounded and the finish of each coil is positive.
One bobbin is magnetized “North” top and the other bobbin is magnetized “South” top.
When the two pickups are combined in parallel the pickups will be magnetically out of phase because of the opposite polarity on each pickup.