What causes a rangefinder to require alignment?

Note: this is a technical question. I've seen many times people saying that a hard shake or a drop can cause alignment issues. I want to understand why.

I have two rangefinders. Both are 10+ years old and both were slightly misfocusing when I acquired them. The Leica's adjustment was easy-peasy. The big screw required a tiny turn, and it was reassuringly tight and solid. No problems since then. My Fuji rangefinder was way, way, way more fragile and finicky: with a tiny screw sitting on a jumpy "leg" i.e. you can't just turn it while looking at the ground glass: the image jumps up/down due to slightest touch of the damn thing.

I have looked at rangefinder diagrams on the Internet, and TBH the device looks fairly simple with very few (basically just one) moving parts. So I am a bit puzzled by what causes them to get misaligned? The Fuji GF670 I was talking about is especially prone to this: it arrived misaligned and it drifted off again after just a few dozen rolls. Is there something else that needs to be tight? Do the screws just turn by themselves? Something mechanical is obviously moving inside. What is it?

RF system is adjustable system. Made to be adjustable in multiple points. Screws and arms, angles. If RF mechanism is well build, it will withstand regular use. If camera gets bumped and build is less quality, arms and angles will drift.
If user is dumb and mounting lenses wrong way, not carefully - arms and angles will be shifted. RF unit is not build to be dumped at the concrete either.
It was not uncommon to read about drifting RFs on Cosina Bessa R series. I'm not surprised. I had Bessa T and on the second roll the screws holding cold shoe for VF became loose. I sold it quick after fixing it without waiting for RF to drift just from regular use.



http://www.angelfire.com/biz/Leica/page10.html

I have no idea. I have two rangefinder and BOTH have crooked patches... the vertical lines of the patches are askew. The are aligned, though, because they focus correctly if I use the center of the patch.

As you can see it has quite a few pivots and sprigs etc that can move over time, some are more complex than others. The little mirror is very small and only needs to be out a tiny bit to throw the range out.
Some cameras like the Contax 3a need quite a bit of work to adjust, including taking the top plate off, some just have a convenient screw on the front.
I just did my Vitessa 1a which not only you need to remove the top plate but you have to reinstall the top plate to check each adjustment, what PITA.

In my experience, I'd say it is a screwdriver in the hands of someone that should not have one. I must have about 20 rangefinders, but the only ones I have ever noticed to be off had signs of being tampered.

I had a Konica auto S2 that appeared to have a misaligned rangefinder. I opened the top cover and found that a pivoting arm that followed a cam movement as the lens was focused wasn't making contact with the cam. The pivot point was a little sticky from being 50 years old with a bit of dried up grease. IOW, it was sluggish, not misaligned.

Best for service RF I was dealing with was on 4x5 Anniversary Graflex camera. Just as entire camera it was conveniently huge to make it works again with nothing but its original parts.