The bus was traveling from Everett to Seattle Friday on Interstate 5, when passengers say cars began honking at them to get their attention.
They heard a loud boom seconds later and started smelling smoke and burning rubber. A small fire erupted in a wheel well in the back.
Commuters were able to evacuate the bus safely before it went up in flames.
Investigators believe freezing in the brake line forced the emergency brake to stay on during the commute, causing the fire.
The exact cause of the fire is still under investigation.