Hey John,
Some issues arose around Spam networks and Tumblr sites skewing the distribution. Over the last 12 months, these two types of sites outperformed (i.e., gained links and linking plds faster) most 'real' sites. This caused sites outside of these to appear to be trending down in DA, relative to the performance of those other networks. This makes sense, because this is precisely what spam networks are designed to do so, and tumblr.com is currently treated as a TLD, so it's easy for tumblr blogs to gain large numbers of linking PLDs (because they're free to create and link heavily to one another).
We do need to get this normalization work added to the index, but we need to do so in a way that does not allow these types of networks to have too much sway in the distribution. We will ensure that the communication of that change is clear and easy to communicate to other stakeholders for users.