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.

Ideally breasts should not look square following any kind of mastopexy surgery, regardless of technique. By ‘square’ I presume you are referring to a flattened appearance across the lower pole, rather than the gently curving, rounded lower pole of a natural-appearing breast. A ‘square’ lower pole shape can happen with both the traditional mastopexy pattern of scars (inverted-T or ‘anchor’) and with vertical scar (‘short scar’ or ‘lollipop scar’) techniques, with or without simultaneous implant placement. It isn’t usually the skin incision pattern that makes the difference in lower pole appearance, but rather what is done with the lower pole breast tissue (or lack thereof) below the skin.
The appearance of the lower pole will be determined both by your surgeon’s preoperative planning and by their intraoperative decision-making. In a mastopexy-only surgery, one must be cautious not to over-resect (remove) central lower pole tissue. It requires a ‘just right’ approach to modification of the lower poles – removing too little lower pole tissue leaves the breast looking bottomed out, while removing too much tissue leaves the lower pole flattened or ‘square’ in appearance.
One must be especially cautious in patients with lower pole hypoplasia – a term that plastic surgeons use to describe breasts where the lower poles are underdeveloped. Most patients with lower pole hypoplasia require little to no removal of lower pole tissue at the time of mastopexy. Often these patients require the placement of breast implants immediately prior to mastopexy to augment lower pole volume.
When an augmentation mastopexy is performed (breast implant placement + breast lift), the lower pole usually is slightly over-tightened - intentionally - so that the lower pole can support the weight of an implant without ‘bottoming out’. The lower poles may appear slightly flattened for the first days or weeks postop, but they will gradually expand over time.
Again, one must be very careful about the amount of lower pole tissue removal and lower pole tightening when performing augmentation mastopexy, as ‘too loose’ will allow the implants to bottom out over time, while ‘too tight’ will leave the breasts appearing square at the bottom.
Make sure you review numerous mastopexy and/or augmentation mastopexy ‘before and after’ photos when selecting a surgeon for these procedures. You should see lower pole contours that are consistently round, natural-appearing and aesthetically ideal. 

The anchor lifts tend to make the breasts boxy and bottom out over time. The lift is not as durable as the newer techniques.The Lejour is the gold standard of lifts. The breast tissue is perky and round. The reason for this improvement in shape is the rearrangemnt of the tissue that fills the upper pole of the breast and adds projection. The other benefit of the Lejour over other lollipop techniques is the improved healing of the vertical scar and excess skin.

You may be referring to "boxy" breasts. This may happen but usually they round out after a few months. If not, a small touch up revision may fix it. See link below for non-square breast lifts.

It's impossible to make square breast.  As the breast heal and settle, they will round out.  Best wishes!