It was completely unexpected. And it will be completely unexpected the next time, and the next time, and the next time...
And because they ran out of gas.
Contrast the SPD's behavior to how the Bellevue police department handled BLM protests. The police chief met with the protest organizers, treated everyone involved with respect, set some ground rules, and did not start a dozen unprovoked attacks on the public.
When all you have is a warrior cop, every problem looks like an insurgency to be put down. The SPD has an incredibly antagonistic attitude towards the public, and is institutionally incapable of de-escalating. It also seems to not actually be subject to any form of democratic oversight - as city council, despite passing many, many resolutions, is unable to reign them in.
So you blocked the police out of a whole neighborhood of a few square blocks, and a gang moving into the power vacuum is a surprise? Come on!