Some time ago on the Debian mailing list there was a discussion (there are many discussion over there ;). That one was extremely interesting, not because of the problem (I don't remember it), but because of the Jörg-Volker. Peetz mail with information on advance aptitude usage:
"I would try the following:
- to find out what is installed
aptitude search '~iapache'
- why it is installed
aptitude why apache2-mpm-worker
maybe this one is only recommended by another package
- and what depends on this package
aptitude search '~i~Dapache2-mpm-worker'
- finally, see what would happen, if it is removed:
aptitude -s purge apache2-mpm-worker"
In the same thread Bernd Semler suggested following command:
apt-cache rdepends $packagename
The original thread can be found here: http://lists.debian.org/debian-user/2011/10/msg01472.html