Her hair was braided in a single thick cable that hung down to her shoulder blades, a way he could not recall having seen her wear it (and maybe worn that way for just that reason).
Why would Mary have taken the trouble of wearing her hair in a new way?
People often wear their clothes, jewelry, hair and the like in ways that signal things about themselves. For example, someone in a suit and tie is more likely to have one kind of socioeconomic background, education and career than, say, someone in faded overalls and work boots. Obviously, there are individual exceptions...but it's a good general rule.
And people sometimes signal change through what they wear. If your college buddy had previously gone through like in suits and ties, and then you see him in overalls and work boots, you might wonder if he's become a victim of the recession and been forced to take manual labor to pay the bills. Or - maybe he's just decided to switch careers.
Likewise with hairstyles. Mary could have been saying: "I'm not the same woman who was married to you for 20 years." That in turn can imply: "I've changed in terms of what I will and won't tolerate. You may be able to win me back...if you change your ways."