Little information about the “why” is available but the back up is quite obvious.
A crash reported north of the Sunset interchange in the northbound lanes is responsible for a traffic back up that, as of 4:50pm, extends past Lakeway Drive.
Southbound lanes are also backing up apparently due to drivers slowing down to observe the scene (aka rubberneckers) since no other incident in those lanes has been reported. This has created a backup reaching nearly to Northwest Avenue.
Monitor the situation in real-time on the Discover Ferndale traffic/commute page.