A dumper can put a buy wall of 500 BTC to make feel secure other buyers, once they put more buy orders around that price, he dumps 2000 BTC which at the end he dumped only 1500.
this is well know for dumpers who dont want to affect the price too much
I prefer that to someone just dumping them without trying to keep the price up a little. Of course, it is in the "dumpers" best interest to do this so he sell at a higher value that way.