It's an assumption based on the fact many of the ships surrounding this sub are being dismantled, and it is at a location where many ships like the battleship Missouri had been mothballed for many years. However, after reading the description of the base, it very well could be taking on missiles.
I agree with Anonymous, I think it looks very much like missile being serviced. There a lot of stuff going on at that location, look at the Google Map. Several dry docks and other operations that are clearly not involved in demolition. But also, it's a Trident sub, and I don't think any (if at all) have been recently decommissioned.
Here's that description:
http://www.globalsecurity.org/military/facility/puget_sound-nsy.htm
http://donshelton.net/djs-srp1.htm