What do you mean back of the tower? Like the physical back of the tower or the back of the card is touching the front of the case.
The card is suppose to touch the back of the case. There are screwholes that will match up that you use to add extra support for the card.
I just added pictures... you see how in the second image on the left of my hand it pushes up against that back part of the tower? The graphics card is very close to fitting in just about a inch or 2 away from fitting in.