If you look at all the pictures where you can see a couple, you'll notice that the wife is always slightly behind her husband when she's walking. That's no coincidence. I've never studied japanese marital customs intensively, but if I remember well, up until those last decades, a married woman would always walk behind her husband, looking slightly down, as a sign of deference towards him. Like many others, this is one of those traditions regarding the woman's status that's actually changing now with the latest generations, although I do not know when this actually started to change. Anyone's got a clue?

But the imperial family being the keepers of traditional Japan, it is not surprising they still conform themselves to those ancient social rules. However, this was the first, and last time Sayako-san probably had to do it; since through that marriage, she has left the imperial genealogy to become a commoner, that rule shall not govern her life anymore