Bitcoin Core Update from 0.13 to 0.15 and still verifying block, descending "new best chain"
Bitcoin Core 0.13.0 did not have SegWit's activation parameters (they were added in 0.13.1). This means that after SegWit's activation, your full node was not aware of it, and thus did not ask for or verify block witnesses.

Bitcoin Core is a fully validating node, so after upgrading to 0.15 it noticed that blocks were present that were missing witnesses, and as result also didn't have those witnesses validated. As a result, it is rewinding blocks, undoing the effects of those post-SegWit-no-witness blocks back to SegWit's activation point. When that completes, it will start downloading the blocks again (with witness this time), and validating them.