Why do I get back pressure from rear passenger shock?

I just installed an air shock on my rear passenger side and tested all the air lines to see if they received air. All the lines checked out so I hooked it up to the shock and when I started the car to let the shock balance out and level the car I got back pressure in the hose and it blew off at a connector in the front of the car. I tried doing it while the car was still in the air and on the ground and I got the same results. The car is a 1999 Cadillac Catera.