Hmm I actually didn't notice the interest for HBD has dropped to 15% until Hivians start to write about it recently. Looking at my wallet and the pending interest, I did a rough calculation and it seems to me that the 15% has being going on for at least a week or two. I remembered it went down to 19% earlier this year as well and then it went back to 20%. For me, am not making changes. It's still 50/50 split between Hive/HBD for my posts.