I'm no pro but I dont think this is good.

Today I was loosening all my header bolts to remove the header for inspection. (It makes noises- checking for cracks) Anyway all the bolts screwed off fine but one- it turned the whole head stud. I'm sure I could have removed the stud from the head just by unscrewing it. I carefully put everything back and used pliers to hold the stud while I used a wrench and put the bolt back on tight. What would cause the whole stud to come loose like that and how do I go about fixing it?

 

I would have pulled it out and looked at the threads.

Chances are it just came loose because the nut and stud were corroded together. Its happened to me many times, and I just usually put 2 nuts on the stud and screw it back in tight. Check the book for torque specs cause I cant remember