From experience, tend to agree with RD Campbell (what a great book).
Remember that directional stability means the aircraft tends to yaw into a slip. This means the slip has to exist before the aircraft will yaw: in a steep turn with a bit of rudder laziness, the aircraft will be slipping in the direction of the turn, directional stability never quite removing slip altogether. Since the aircraft will generally be slipping into the turn, the upper wing will tend to stall first.
*generally inserted to allow for the case of a high power setting / low speed causing yaw to the left, regardless of direction of turn.
Also pilots stalling in a steep turn commonly do so due to a low airspeed rather than high load factor, and if this is the case they may try to instinctively maintain the correct attitude with rudder instead of reducing bank or adding back pressure. This will also contribute to slip.
O8
(Edited to add the bit about rudder misuse.)