Too high a temp is the number one cause of Case Bursting! Indirect heat is the best choice with a Quick sear if needed.
Think about your Recipes also. Do you add more water to Pork Sausage than Beef?
Fat Content and Emulsification. High Fat well mixed and Emulsified with the Water, will hold more moisture than Leaner Sausage. Emulsified Fat and Water plumps and expands verses loose or unbound water that Evaporates.
Do you add Binders to both Pork and Beef Sausage? Binders hold moisture when added, plumping, verses leaving them out.
That's all I can think of off the top of my head...JJ