More than 20 coins are/were delisted now or recently from cryptopia, and their listing fee went from 0.3 to 8 btc or something like that.
Sounds more like they have difficulties to manage a lot of coins.
Cryptopia is a small exchange, Yobit has 6x its volume.
Regarding sigt, the old and self-claimed "community leader" team was here to scam obviously, but after they got fired, we didn't see fast and massive pump and dump anymore.
I think the price is growing because of the end of the PoW phase and masternode discussion, and also because of all the fud : when there is so much fud on a coin at launch, you can be quite sure it will have a future (check the zcash thread for the laugh).