The pipe you are using, Does it have the rubber o-ring gasket?
If that is what you are using. It takes the pressure in the line to tighten the seal.
The one that came apart may have not been together good enough.
For this type of pipe to seal right, all connections must be flush together, when pressure is applied, the entire line will move(stretch out) as it tightens up the seals.
If we are talking about the same pipe.