From KIRO 7 Eyewitness News:

A propane tank that fell into a piece of machinery caused an explosion at a metal recycling plant in South Park, blowing holes in the structure and buckling doors and walls. The blast occurred at Independent Metals at 747 South Monroe Street in south Seattle shortly before 8:30 a.m., drawing a large response from the Seattle Fire Department.

I live a couple blocks away, and it sounded like someone jumping on the roof. Here's my favorite KIRO-supplied detail about the cause of the blast:

Fire officials said a propane tank exploded when it fell into a shredder.

No one was hurt.