What was the impetus for the creation of the third major political party system?

I am assuming that you are referring to the party system
that existed in the time leading up to the Civil War.  This was the one that had the
Democrats opposing the Whigs and which eventually broke down as the Whig Party split
over the issue of slavery, as described in the link
below.

The impetus for the creation of this party system
was the fact that there were splits arising between the "common people" and the richer
elites.  The common people moved towards the Democratic Party during the Jacksonian Era
and the elites set up the Whig Party.  The Democrats wanted more democracy and they
disliked big businesses and high tariffs.  The Whigs wanted the government to work to
improve the economy and to help manufacturing.

This party
system broke down in the 1850s as the issue of slavery split the Whigs into Northern and
Southern wings.