What would cause an outer tire rod-end to loosen up after you torque it to manufacturer recommended torque, using wrench?

Which end are you referring to? If it's the tapered stud that's coming loose, it is because one came loose previously and wobbled out the hole in the steering arm. On your car that arm is part of the steering knuckle. That means the knuckle and outer tie rod end must be replaced. If you try to reuse the outer tie rod end that previously cam lose, that stud is also worn. It will come loose again.

A similar problem can occur to the threaded adjuster sleeve. If the locknut came loose previously, normal driving will cause a hammering effect on the threads. That can be bad enough for the two parts to completely separate from each other.

Another thing I ran into once was someone tried to be conscientious and coated the replacement parts with anti-seize compound. That must never be used on lug nut studs or steering and suspension parts as it removes the holding power of threads and the clamping power of bolts. There must also be no grease or other type of lubricant on the tapered studs. Those require the friction fit to hold them tight.

Does that sound like something that may have happened? If this problem is occurring on just one side of the car, and you have to replace the tie rod end, you can get the alignment real close by observing the steering wheel is straight now. Once the parts are replaced, tweak the adjustment of the new tie rod end until the steering wheel is straight again. That will put that wheel back where it was.

There was also an issue some years ago with the tapered studs on some tie rod ends. They tried to use a part that fit one model on a different model, but in some cases they ran out of threads on the tapered stud. The manufacturer supplied two steel washers to be added under the castle nut to take up the extra unthreaded section. That worked just fine, but if you weren't aware of the need for those washers, the nut would run out of threads and reach the specified torque value long before it pulled the stud into the tapered hole. You can see that if you look at the installed stud before the nut is installed. There should be at least one or two threads inside the hole.

Check out this article too to see if there's something I overlooked:

https://www.2carpros.com/articles/tie-rod-end-replacement

Let me know if you figure this out.