This afternoon, on Poloniex, DCR was rallying with plenty of momentum to break through .0100 for the third time of the day. Just as the price hit .00998, a trader put 550 DCR for sale for .009999. A few coins sold but the rally wouldn't buy thru such a big amount of coins (is this called a buy-wall or a sell-wall?) and the price fell. Then, what appeared to be the same trader lowered his price, apparently still trying to sell, and in this chased the price down further. As it turned out, he only sold a few coins because he drove the price so low. Maybe I need to stop watching Poloniex.
It's indeed called a sell wall. I'd say a rally that gets deterred by a reasonably small sell wall like that wasn't going very far anyway. It just shows that at the moment that's what the market decided. With patience more organic growth will happen over time, and that's probably better for Decred than short-term spikes.
I should start watching the exchanges more closely again. The ups and downs are all that meaningful (since it is just the longer term trends that matter), but they can be exciting.