sounds like the temp was to high.  If you smoke it at a lower temp it will take longer to get the interior temp to 165 but the outside won't be crispy. Or else they were to close to the heat source.

Sounds like over-cooking for sure, but there may be some other voodoo could be going on.

Some questions:
Did you use insta-cure?
Did you use a seasoning kit or your own recipe? If your own recipe, did you add a binder?
How coarse or fine did you grind the meat?
Did you add beer to the mix?

I preheat my smoker to 190, then pull it back to 165 when I hang the sausage.

I've jacked up a few batches. It happens.

The recipe looks good, other than not having a binder of some sort (soy protein, whey). That will certainly make it crumbly. At this point, I'd say the lack of a binding agent is your biggest issue along with the lengthy smoke time.
I started using the seasoning kits from Sausage Maker and they turn out great every time. You can use them for venison, just substitute the pork in their recipe for venison and pork fat in a 50/50 ratio (or whatever your preference).


SausageMaker.com

The crumbly texture of the finished sausage can be caused from not enough fat content in the sausage to begin with or cooking the fat out at to high a temp. The lean to fat content should be at least 80 to 20 percent. Be sure to check your thermometer for accuracy.

Did you bloom the sausage? This means to spray cold water on the sausage after removing it from the smoker.This helps to tighten the casings up and stops the cooking process.

I don't think natural casing are good to use for smoking. Should use these instead.Smoked Collagen Casings

I have used the collagen casings for snack sausages, think Slim Jim's.

They worked fine for me, but I wasn't hanging the sticks, they were laid out on racks. The collagen casings are tough and allow you to really stuff the meat into the casing without worrying about the casing blowing out.

I've used hog casings on smoked sausage with good results numerous times.  I've only used colalgen casings on things like sumemr sausage.

Cold smoke them next time. Longer smoke, better flavor. Mmmmm good.

Did you keep all of your meat below 40 degrees while grinding, mixing, stuffing? I had what you describe happen once.

If you allow the fat to get too warm it will "break". When you cook the sausage the fat liquefies and leaves the meat instead of staying with it. I made a batch of brats one time that did this. the casings looked like meat floating around in liquid. When I cut the case the fat ran out, the skin shriveled and the meat was dry and mealy. It sounds like this happened to yours only you let it cook until the cases split and the fat all ran out.

After that experience, I do just like my book says.
Put all bowls and grinder parts in the freezer an hour before beginning
freeze all meat and fat until stiff but not solid
grind into a metal bowl sitting in a plastic bowl full of ice water
if the meat starts getting too warm at any point (over 40) place it into the freezer to chill it back down.
ETA: I find it easiest to make sausages outside on the deck when the temps outside are below 40.

I used to get it from my local mom and pop grocery, but they got bought out and quit selling it.
Now I get it from a local meat market.

Since they don't raise fatty pigs any more, pork fat has become hard to come by. The local butcher used to sell me some, but he quit. He says he is having to buy extra fat to supplement the pork butts to make sausage, and the fat costs him more than the pork butt.

I recently bought a whole hog from a friend that raised them, to be a little fattier. When I had it butchered I requested that they save ALL of the fat so I got about 10lbs of loose fat bits.

Yes, you have to beg your butcher for the fat.

Also, when grinding the meat and fat, put it in the freezer for an hour or so before hand. Not enough to freeze it solid. It will grind nicer. I also put my grinder and plates in the freezer before hand to get them as cold as possible.