Probable cause - downdrafts that exceeded the maximum climb capabilities of teh aircraft.
summary:
http://www.ntsb.gov/ntsb/brief.asp?ev_id=20081007X17184&key=1
full narrative:
http://www.ntsb.gov/ntsb/brief2.asp?ev_id=20081007X17184&ntsbno=SEA07FA277&akey=1
Brian