The backpack was apparently full of electronics. (Which I take to be phone, tablet, notebook.) Backpack catches fire on Southwest Airlines aircraft at Chicago Midway International Airport.
The fire was put out and contained to the cabin on the aircraft, according to Chicago Fire Department. They also said the incident was due to either a battery or charger.
And of course bomb and arson units were called out because terrorism is the new normal.
Lithium-ion batteries catch fire. It is one of their many qualities.