HBD is not a true stablecoin, it is basically a token that grants you getting Hive that is worth one dollar. If there was no Hive Stabilizer that keeps it more or less pegged to USD (and reversed conversion) it would range quite a lot.
Speaking about reversed conversion, it also grants an unlimited amout of HBD in existence. You can anytime convert your Hive and get HBD. However, when Hive price is low, you obviously get less then if Hive was worth say 3$.
How exactly has the 20% APR added to liquidity? HBD in Savings is not liquid, it is staked.
I wrote down four reasons that should lead us to reconsider the current rate earlier this week. The post has over 100 comments, perhaps, you may find it worth reading (both the post and the discussion).