Have you seen this before? Enamel primer issues

Yikes! That is some nasty cracking indeed. I have gotten away from using rattle cans about 20 years ago or so, so I can't help you with this issue as far as why. But I can recommend that you switch to using a primer that you can airbrush on yourself. Tamiya, Mr Surfacer, Humbrol, Testors... You will have a LOT more control over the situation.

You didn't do anything wrong. It's the paint.

Looks like to me you laid the primer on too thick instead of in several light coats. Humidity could be another possibility too. I use Rustoleum Grey Primer in rattle can from Walmart but not the automotive kind though.

The most common reason for cracking is applying the paint too thickly. It's possible in this situation that since you were down to the end of the can, you had a mixture left that was more paint than whatever they use in rattle cans to keep the paint thin and help it adhere (let's call it, oh I don't know, "paint thinner" for lack of a better term). That imbalance could have caused the issue, as you would be applying more paint and less thinner. Just my theory.

Thanks guys! Since it did this same thing on three different models, (but not as bad) I kind of wrote-off it being from the bottom of the can.. It seems like where it was laid too thick (due to shaking the can while spraying trying to get the last drop!) is the worst spots. I would love to find a good primer that can be airbrushed, I used some MM Enamel primer with the airbrush and had no such problems on a 1:48 PBY Catalina I am finishing up. The enamels are so damn stinky, I was forced to spray them outside with a spray can now that it is warm to avoid the whining of the wife and daughter. I have since added a bigger blower to my paint booth, but have not vented it to the outside yet....

I airbrush Tamiya bottled primer (lacquer) and AK acrylic primer, and have never had an issue with either.  Tamiya has more bite but AK is odorless.  Avoid Vallejo primer because it is not sandable.

Tamiya primer in bottle or rattle can is a great product, I especially like the can for large areas. For smaller stuff, I use Alclad primers. They're lacquer based, so you'll need good ventilation, but they airbrush really well, stick tight and sand easily. A 4oz. bottle goes a long way.

I think that is lacquer not enamel.  Lacquer is brittle and cracking of lacquer may be due to a number of reasons.  As someone else suggested, putting it on too thick is one, attempting a recoat before first coat is dry is another, applying over an incompatible paint is a third.