Ironworker71 Protein extraction is when the meat binds together. Your fat is going to start melting almost no matter how you cook it. Having proper protein extraction is what prevents what happened to you, the fat melts, but has no way to get to the outside of the sausage because it is surrounded by proteins that are stuck together. It is how you can get nice fat chunks in summer sausage and snack sticks. If I had to bet I would guess that is the cause, because your smoke schedule seems fine to me. Like Jamieson22 says, it could be the fluctuation but even if it did rather large ups and downs the temps should still have been okay.
Sure Gel Meat Binder is a great way to make sure this doesn’t happen in the future, not expensive and adds a large margin of error to the mixing process. Also, doesn’t make it harder to stuff like Soy Protein Blend does.
Do you happen to know what the temp of the meat was when you were mixing it? If meat gets too warm it will not allow the proteins to be extracted no matter how much you mix it. Years ago Austin and I did a video, you don’t have to watch the entire thing but here is the relevant part of how sticky the meat should be https://youtu.be/wStH-RtQUY8?t=82
If you are sure that your meat was mixed well then give us as much detail on the entire process, what was your meat block, fat content, how did you grind it, how long did you mix it and how much water did you add? During the smoking process did you add any humidity?