This is quite normal. A lot of people are expecting a rise in price when a coin is listed on Poloniex.

Yes I understand. But do you think that Polo uses this to their advantage?
Do you think they fill their bags with that coin before listing it?

Of course they can do that, because they will surely make some profit on it, but I don't believe that they are doing it.

can you guys help me understand what wash trade means and how they benefit from it?

You can call it "fake" trading (i.e. buying and selling something, pay just the fee for the trade and create artificial volume).