My NRT-SLC segment was very full, >90% I guess, but return was around 80% which the FAs said was higher than normal for the return.

I am guessing the japan originating passengers are making good connections to certain places (like DEN, LAS, etc.) via SLC on the way there, but they are visiting other cities, like LA/SFO of east coast and therefore the return isn't routed thru SLC. Might only take some tweaking of the flight times for LAS-SLC or whatever destination people are going to in order to make the route/connections more appealing.

Perhaps this is a baseball season route. starting up beginning of may and ending oct. 1st.