What is the strategy behind a sell wall? I understand the idea behind 'bid walls', but on Kraken, prices are being kept low through the use of 'sell walls'.  The volume on this exchange is too low to have a chance of getting through these walls, even when prices on other exchanges are skyrocketing. Every so often, when the price falls a bit, up comes a new 'buy wall' keeping it there. The price on Kraken falls when other markets fall, but gets stopped at the wall on the way back up.  I am curious to know what the strategy behind this tactic could be.




Added 21 Nov for elucidation

On this small volume exchange, a 30 BTC ask equals 10% of the 24hr. volume.  It is the equivalent of a 4500 BTC ask on Bitstamp.  It alters the way bidders approach the market.  When you see such a wall sitting just above the current price, you can be sure the price will not be rising beyond it for at least the next few hours. 

I watched this person re-position this block four times over the course of the day.  Each time, lowering it to sit just above the current bid price.  It was altering the dynamics of this small volume exchange.

I am not asking for you to read anyone’s mind.  I was baffled by this behavior.  I had not seen this strategy before.  I thought that there must be some method to this madness, but I could not see what I might be.  I thought that somewhere in the collective experiences of Stack Exchange there might be someone who was familiar with this strategy and might be able to provide some insight, not necessarily on this person’s behavior, but on the theory behind the strategy being employed.  Assuming, of course, that there is one.
A large investor might be trying to move the market.

Sell walls create an impression of a strong supply. This makes some people, unwilling to wait for the wall to break down, pitch their sales offers below the wall. By moving the wall closer to the current bid again, a large investor might be able to move the market to a small extent and then buy at a lower price. The investor's goal is to move the market, not to actually sell his supply at that price, so when the price gets too close, such walls often disappear or move to a higher price. Especially, if you see the same amount popping up at different prices, there is probably someone trying to move the market without actually fulfilling orders or bids.

Another type of sell wall is composed of many combined sales offers at certain landmark prices, e.g. $600. These usually don't move around.