like I mentioned above, it is speculated that they are the ones who are printing the tether, then use that to buy bitcoin. They are then probably hoping that whoever receives tether for selling their bitcoin does not immediately want usd, but instead is happy with the tether as it is 'backed 1:1'. (Again, I have to stress that this is a theory i read somewhere on the internet, while lots of things add up, it may also be completely off.)
They 'pump the market' because buying bitcoin increases its scarcity, thus raising the price.