Is there a way to find out from which Package-Installation a certain package was installed as Dependency?
You could check /var/log/apt/history.log for a log entry.
Also, there are several ways to check for reverse dependencies:
Will show you all packages that depend on
This might require to install aptitude
if everything else fails, try to remove the package in question and look at which other packages would be removed because of your decision. Remember to answer "n" to the "are you sure?" question.