Comment 3 for bug 1408031

apt-get returns 1 when you press n, so ppa-purge goes onto to try the fallback (aptitude), seemingly that runs without even asking.

The fallback was meant to catch the case where apt-get gets confused by the revert list not being 100% accurate. That should be much better in the new version in Xenial, but its still possible to hit that case, if there are packages with cross-ppa dependencies for example.