From the Kitsap Sun:

Sonic booms from two F-16 fighter jets spooked people between Everett and Olympia this afternoon, according to Allen Kenitzer of the regional Federal Aviation Administration office...

Kentizer said the F-16s were scrambled in response to a security scare involving President Barack Obama's visit to Seattle.

And the Seattle Times:

The F-16s were being "scrambled" because someone flew a plane inside the "temporary flight restrictions" area set up around Boeing Field, [Kenitzer] said.