I don't believe it's obvious despite the fact PFF charged Shields for the reception/TD.
This play was discussed at some length in the "Projecting 2015 Season" thread starting at post #123. The link from that post, which provides a complete look at the play is reproduced here:
http://insidethefilmroom.com/seattle-seahawks-shredded-packer-college-style-spread-offense/3/
I don't think it can be ruled out that Shields was doing what he was coached to do. If Huard is correct in noting the Packers corners were cheating toward run earlier in the game, that notion is supported. This could be one of Capers' data-driven moments vis a vis the frequency of Wilson throwing the ball from a read option keeper to the left side that Shields internalized.
Further, if Shields has the option to read run for edge contain, whether he read right or wrong, Dix should have anticipated that Shields might take that action. In that case, Dix is to blame for being late to react. Shields is 5 or 6 steps post-snap into run defense before Dix reacts to the open receiver, not to mention Dix blew the tackle.
In the final analysis, this play has a similarity with the playoff fake kick...the Seahawks identified a tendency and exploited it.
Assuming Shields' move is not a complete ad lib, it's worth noting that Shields having that option with 8 already in the box says something not so positive about how those 8 players are schemed.