day 6

[Hashira]

why did takanohana have to switch to the east side when he faced dejima who is west rikishi? i figured the lower-ranked guy would switch

[Zentoryu]

Well, the east side is the more prestigious, so when two west ranked rikishi face each other, it's likely that the higher ranked rikishi gets the more prestigious side.

[Tony]

But day 6, Takanohana fought Tosanoumi who is maegashira 1 East.

Me to, I didn't understand!

[Kotoseiya Yuichi]

(...) so when two west ranked rikishi face each other, it's likely that the higher ranked rikishi gets the more prestigious side.

Higher ranked rikishi is allowed to keep his tozai side. Higashi sei-yokozuna (now Musashimaru) always uses the eastern side, nishi sei-yokozuna (now Takanohana)  always uses the western side.

If there were a third yokozuna (on the east side), he would always use the eastern side (even when fighting the western yokozuna who would be of higher rank than him) EXCEPT when he were scheduled to fight the eastern sei-yokozuna.

Hmm, I'm explaining this in a hard way. In short, the higher rank rikishi always keeps his tozai side and the lower ranked must change sides for the day.

As for the original question... it's a very good one. I believe it must have something to do with eastern yokozuna's total absence this basho. Western yokozuna probably assumes the eastern position. But why would he actually...? This is a tricky one. (What is that?)

[Guest Kyukenzan]

Wow, I just recently got interested in sumo while studying in Japan. Seems I have a lot to learn, I had no idea whatsoever of this east/west-side system. Nice to know at least that the sport has a lot of hidden depth  (Happy...)

[Zentoryu]

I believe it must have something to do with eastern yokozuna's total absence this basho. Western yokozuna probably assumes the eastern position. But why would he actually...?

As good an explanation as I've heard. Perhaps it's also the preferance of the Yokozuna in question?

[Doitsuyama]

I haven't an explanation. Just wanted to add that this phenomenon actually started on day 3 with Kyokutenho's fusensho against Takanohana. This was the first day with Takanohana on east side, even if he wasn't on the dohyo. The first two days were correct with Takanohana on the west side.

This really looks like an ongoing error since day 3.