Immediately after the Super Bowl win discussion turned to keeping the Seahawks' nascent dynasty together, and Ian Rapaport reported that they would prioritize Earl Thomas over Richard Sherman for an extension. There wasn't much elaboration, and I'm going to assume that means extending ET before RS, not ET instead of RS (I don't see a scenario where the Seahawks ultimately fail to retain both).
Here's my explanation (wholly based on contract details, not on-field ability or contribution) for why pursuing it in that order makes sense: it's all down to the distribution of salaries in their respective positions (plus a little bit about their current deals).
Cornerback has a major outlier in average annual salaries among top players: Darrell Revis' $16m/yr. It then drops down to a bunch of guys earning about $10m/yr--what this means is that the franchise tag value (average of top 5) is a long way beneath Revis' salary. If Sherman insists on being paid as the best corner in the league (and interprets that as meaning beating Revis' deal), Seattle can save a lot of money by franchising him, possibly even for multiple years.
Sherman's first-year franchise tag (if they let him play out 2014 on his current deal and then tag him for 2015) wouldn't be much more than $10m, and even a second-year one (for 2016) just over $12m (can't predict these exactly because it depends on other new deals but also on whether Revis is cut at some future point). This would also save a lot of money in 2014 (keep him at his $1.4m cap number) where in an extension they probably have to pay him a lot more next year.
Even a fairly team-friendly deal that doesn't try to beat Revis' might pay him $12m/yr starting with 2014. But instead of $12m/$12m/$12m over the first three years of it, the Seahawks could pay $1.4m/$10m/$12m over the same years by letting him play out his deal and then tagging him twice (total savings of more than $12m), and then still negotiate a long-term deal after.
For Thomas, in contrast, top safety salaries are a lot closer: Polomalu at a little over $9m/yr and then Berry/Goldson/Weddle around $8m/yr. So a franchise tag for him would probably be around $8m (and something like $9.5m for a second one), but you can also hopefully lock him up long-term for "top safety money" at not much more than $8m/yr. And since his cap hit is already $6m for next year, it doesn't cost much to pay him early (say $8m/$8m/$8m instead of $6m/$8m/$9.5m on the franchise tag).
Now the beauty of this approach is that once Earl is locked up (and Bennett/other FAs dealt with), you can come back to Sherman and try to use the leverage of the franchise tag to get a more friendly deal, but you still know that ultimately due to the distribution of top CB salaries you can keep him at a very team-friendly price (relative to the Revis deal) under the franchise tag (which doesn't provide any real advantage in terms of retaining Thomas).