bbPress grabs the 40 most popular tags. You probably have fewer than 40 tags, so it’s grabbing a couple tags with 0 topics in them.
This will clear up on its own as you accrue more tags. Otherwise, you can manually reduce the number of tags bbPress shows in the hot tags area. See this post about customizing the tag cloud.
… or in the “site management” admin tab you can “delete tags with no topics”.
That makes it easier. Thanks earlgrey.