It is alleged that Tether is being conjured out of thin air. They are being issued with no money backing them. Backing is important because each Tether is supposed to be backed by $1 USD. These Tethers are also alleged to being transferred to Bitfinex, one of the biggest cryptocurrency exchange in the world, who then uses these Tethers to somehow buy more Bitcoins directly or indirectly. This implies that Bitcoin's price is being artificially propped up. Why would that be bad? Well, imagine if someone was counterfeiting dollars and started buying houses in your area.