There were mechanisms added in the last hardfork to help maintain HBD price stability. For example, if the price goes up, people can convert Hive to HBD and sell the HBD at a profit, which should bring the price down. Conversely, if price goes down, then people can buy HBD cheaper and hold the HBD for interest and in anticipation that its price will rise (there's also the option to convert HBD to Hive and use the Hive to buy HBD).

The hbdstabilizer additionally helps keep price stability.