Why is it that breasts appear square with an anchor lift? Is it the PS technique that makes it look that way?

You are quite correct.  The boxy breast can result from the anchor lift.  This is due to the lift being relatively tight in the region below the nipple compared to each side. Often, that area stretches over time, thereby reestablishing a more rounded lower pole.  This phenomenon usually occurs when a skin lift technique is utilized.  Because skin easily stretches, surgeons compensate for this stretch by making that are relatively short in anticipation of the stretch.  This is why one, also, often sees a bit of skin breakdown in that area where the vertical and horizontal incisions intersect.       In the vertical breast lift technique, where the skin minimally plays a role and over-tightening is not necessary, this boxy phenomenon does not occur.  I prefer this latter technique, as I believe not only does it produce a better shape, but the incision quality is superior as well as its durability.Ary Krau MD FACS

The anchor pattern breast lift does not in and of itself result in square breasts.  This is a function of design and planning.  Look at the surgeons before and after pictures to identify a pattern.  If you like the results an anchor would be fine.

The anchor lift produces maximum tension on the
inferior incision horizontally, causing boxy shaped appearance.The tension is determined by the width of the
vertical excision.The anchor technique
was developed in 1956 and often results in nipple numbness, inability to breast
feed, inadequate lift and recurrent ptosis.For that reason, a new technique was developed called The Ultimate
Breast Lift™.The breast tissue is
reshaped creating upper pole fullness, elevated higher on the chest wall and
more medial to increase your cleavage.This technique avoids the ugly vertical scars of the traditional
technique, maintains nipple sensation and the ability to breast feed without
the square appearance.Best Wishes,Gary Horndeski, M.D.

Both can look round.  There is a confounding variable here and that is breasts with a tremendous amount of droop may require the anchor lift and these breasts typically produce a flatter less round appearance no matter what is done.Kenneth Hughes, MDLos Angeles, CA

Anchor lift do tend to look a little boxy at first, but round out over time. It is hard to get photos many months or years later. Lifts with implants are less likely to have the boxy look.