I'm not very knowledgeable with these matters.
Is a sell wall simply a method of keeping prices down for a while or are there other possible strategies behind them?
Well it's usually used to manipulate market to either keep the prices at the same level or make people lose confidence and push it down.
I don't like when this is being done, but it's up to the traders. They don't really care about a coin, they just trade for profit.
Now, while looking at the marketcaps today I must say the day has been good. Most coins have had some gains, with DRK having over 20%.
I suppose somebody may really want to sell the 15k DRK, and is applying some kind of a "storm the castle" approach: in an uptrend, rather than layering the sell offers in multiple smaller amounts, they challenge the buyers to attack the wall. As it starts to get chipped away, the buying may accelerate until it falls. If that works, then they may have set their ask too low!
However, I tend toward the other explanation that somebody wants to hold the price down and accumulate more, now that they see the action is hot on drk