tbone Glad that the taste was great! If you are using a non-stuck casing then yes you do want to perforate it, this allows for the sausage to bleed out of the casing and then the casing to form tightly to the meat. What sort of casing were you using? If it was a fibrous casing you still shouldn’t have had this issue but if it was plastic I can see that happening. The time it took to get there seems pretty high but you weren’t in any danger there, the cure had you covered.
It sounds like too much water was used in the mixture, I know you used carrot fiber, how much water did you use for your 10 lb chub? I would suspect that the correct level of protein extraction was not reached, the water should have been bound up in the meat better than that. Now, I know that some people reading this will be saying to themselves, or maybe even out loud, there he goes blaming protein extraction like he ALWAYS DOES! Well, that is because it is so often the issue with any cooking problems you might have. Seriously, if you are making a cured product, once you have gotten what you think is good protein extraction mix for another 3 minutes or so, it will help!
I’d also try using a probe thermometer and check the actual temperature of your smoker. If you set it for 150° is it actually there or did it only reach 140°, the easiest way to do this is with a probe thermometer system like the GrillEye or the GrillEye Pro Plus, they come with a clip that allows the probe to not touch the metal so it will give you an accurate reading.
So, in short I think you are right in what happened though, I think the meat released all of the added water and then that created a barrier and it was having problems passing meat through that and then into the center of the meat. That should never have happen to that extent though and I think protein extraction was more of the culprit!
Hope this helped, let me know if you need anything else.