The passengers were exiting Southwest Airlines Flight 570 from Phoenix to Seattle when the jetway slowly lowered.
"What happened was there was a mechanical failure that made the jet bridge drop six to eight feet. There were no injuries," said Christina Faine, an airport spokeswoman.
The passengers who had not yet exited the plane had to do so using a stairway that was brought in, she added.
"We're thankful to report that there were no injuries today at Sea-Tac International Airport following an issue with a passenger jetbridge related to flight 570 that arrived from Phoenix," Southwest Airlines said in a statement. "Roughly 60 customers had deplaned when the jetbridge suddenly lowered by approximately six feet due to an apparent mechanical failure. The remaining Southwest Airlines customers deplaned from another door near the rear of the aircraft via airstairs. We're working to accommodate our affected customers as this aircraft remains out of service pending our inspections."