I guess I'm behind the times since I've never encountered a door strut that actually "assists" in the opening direction. Seems to me that most are designed to assist in keeping the door closed. This sounds like something that's meant to work like the lift on a rear hatch, but even those don't usually open violently as you describe.
Maybe the strut itself is defective, or just plain too "strong" for the weight of the door?
" Not every mind that wanders is lost. " With apologies to J.R.R. Tolkien