Force is a vector, with amplitude and direction. If you've ever taken a bike or car around a corner it is a familiar concept. I am not a speaker designer, though have done some, but basic linear mechanics (remembering my one course in mechanical engineering from way back when) will show you why "wrinkles" in the surrounds are needed/helpful since the stress on the surround during excursion (movement) is not equal when you change direction like that (from straight to semicircular at the ends). A circle applies stress evenly at all points (ideally) but when you have an oblong or rectangular driver then stress on the surround comes at different directions where it changes orientation. The "wrinkles" allow the sections at the end to flex more without having the surround non-linearly modulate cone movement and add more distortion.
FWIWFM -Don
FWIWFM -Don