From the release notes
If we detect that any of your packages were affected by the retargeting or upgrade, we’ll produce immediate build errors to let you know. In addition to the immediate build error, we also persist a requireReinstallation=”true” flag in your packages.config file for all packages that were affected by the retargeting, and each subsequent build in Visual Studio will raise a build warnings for those packages.
Essentially, NuGeT is automatically flagging packages which conflict with your project target or version