The story on BikePortland has more details, but basically, it appears that Christensen had a close call with a cyclist on the non-bike-lane part of SE Hawthorne between SE 20th and 40th. He was apparently shaken up after the incident, and posted a photo of the cyclist (taken by a bus rider) with the headline, "PORTLAND! KILL THIS BICYCLIST."
Thank you for putting your trust in my training and my willingness to throw people around my bus by violently breaking. However as a holder of your life I think now after long hours of contemplation I shall exercise the death option.
TriMet placed Christensen on leave immediately after finding out about the post, which has since been taken down. The agency also notified the police and the district attorney's office.