Why does aptitude markauto uninstall my package?

From the aptitude help:

aptitude 0.6.8.2
Usage: aptitude [-S fname] [-u|-i]

.. snip ..

markauto     - Mark packages as having been automatically installed.
unmarkauto   - Mark packages as having been manually installed.

.. snip ..

So why when I mark my package as being automatically installed, does aptitude attempt to uninstall it?!

# aptitude -s -y markauto python-pip
The following packages will be REMOVED:
  python-pip{u}
0 packages upgraded, 0 newly installed, 1 to remove and 0 not upgraded.
Need to get 0 B of archives. After unpacking 468 kB will be freed.
Would download/install/remove packages.

Thanks!