One factor that works against the peg of HBD is the fact that exchanges do not list it as a stablecoin. Take DAI for example, if you look at the trading pairs on Binance you can see that Bitcoin is priced in DAI and not the other way around (same with USDT). On the other hand HBD is priced in BTC, so when the latter goes down the price of HBD measured in USD goes down as well. When BTC goes up in price people buying it with HBD make it look like the dollar value is also going down.

Because of this unit bias average traders see HBD as just another altcoin but the savy ones know better. They take advantage and buy it at a discount, convert it to Hive and make a profit. Rinse and repeat. Of course this works to push the value close to the peg at the expense of Hive.