Nope. It is most probably caused by operational stress - rudder assembly is moving, fuselage is also working (compression and decompression cycles on take off and landing, thermal expansion and compression). I bet they don't just put red Loctite on it to keep it from getting loose. My bet is design flaws, not manufacturing or QA.
EDIT: I saw the pictures of bolts with pins and bolts without pins. The ones with pins cannot get loose, the others can. Let's see what happened.
I don't know about you, but in my industry, "QA" also means extensive testing to ensure that part/assembly/etc doesn't break with expected operations. So, yeah, from where I'm standing, this was a QA problem. Something did not get checked or tested as it probably should have.
The 4 restraining nuts and bolts on the door have a cotter pin like mechanism to prevent them from loosening. If assembled correctly they cannot loosen unless the pin fails.
EDIT: I saw the pictures of bolts with pins and bolts without pins. The ones with pins cannot get loose, the others can. Let's see what happened.