There has been a lot of discussion about this in other threads.. The short version is authors will get paid more if HIVE goes up too. The original design of the two currencies was for HBD to be a controlled stable currency and HIVE to be driven by market forces. The witnesses and stakeholders who are supporting this effort would like to see HBD stable and HIVE go way up.

There is more info on HBD in this post as well. It was written back when it was called "SBD" but all of the logic and economic principles still apply.
@timcliff/sbd-explained