PDX was down to no international flights (maybe a flight to Mexico, but I think even that was gone, or was either seasonal or not daily); so the city was desperate to get some sort of international service (or they'd have to change the airport name). A big pitch to lots of airlines, and LH started service about 3 years ago, with lots of guarantees (I think half their capacity the first year was prepaid). Rather surprisingly several German companies have operations in the Portland area, also Star has pretty good feed at PDX, so it has been moderately successful for LH (which also operates to DTW so they are used to having specialized markets).

Rather unfortunate that PDX built a great airport and no-one came (or rather it didn't make it as an international gateway; DL is rumoured to have lost a billion dollars in trying to make PDX work).