xem at poloniex have fake sell wall
to have down the price
but if you see the charts until 1900 is no big wall
simple someone put a lot of xem at very high price
nem soon will hit 3000 for sure

Can you explain why do they do that?
I've seen it mentioned with quite a few coins that someone is trying to keep the price down but I never understood how does the low price benefit them?
That's done when whale(s) wants to accumulate more coins for cheap prices. When they have enough of them or the buy pressure become too big they usually remove the big sell walls and let the coin fly (taking profits on the rise obviously).