http://losangeles.cbslocal.com/2017/12/27/passenger-incorrect-flight/
In the article it is mentioned that on board were 2 passengers who are brothers, one had an ANA ticket and the other one a United airlines ticket. ANA and United share trips.
The flight was 4 hours into an 11 hour flight. It seems not logical to me at all that an airline would return that flight, wasting the time of their clients and money on fuel (for 8 hours).
I would understand it if it would be returning because of a missing passenger with baggage (because of the risk that a bomb would be in the luggage). But not if you have one more passenger on board, I would say no problem at all... you get to Tokyo and your luggage stays in LAX... period.
This seems more like they wanted to intercept those brothers and have them back in the US. As if they were wanted for something... I am not aware of any regulation regarding returning and spending half a million USD in fuel for nothing...