HBD is backed by HIVE.
There are multiple mechanics to keep the price around the dollar, like conversions HIVE<->HBD, the internal market and the hbdstabilizer.
That said, outside markets where HBD is listed, especialy the Upbit exchange, can increase the demand for HBD and temporarrly increase the price, or the oposite up until arbitradge is made. You can see on Coingecko how was the price in the last four years. Also if the HIVE prices drops extremly low beyond the debt levels HBD can depeg by design.