Now this is something I’ve never had before.
I was cleaning my bike when I suddenly saw that the non-drive side bottom bracket cup was moving in the frame. While riding I had not noticed anything.
The bearings are still fine, but the bottom bracket cup sits completely loose in the frame. I can just take it out and put it back in without engaging the thread.
What can have caused this?
I hope the frame is still ok and I can just install a new one…
Two critical things that destroy frames: loose headset bearings and loose bottom bracket cups. You will likely not be able to reuse this. In the old days we would try to rethread the BB to Italian as it’s slightly larger than BSA. I don’t know how successful you’ll be wth that approach. However, you can get BBs that tighten against the opposing cups as opposed to using BB shell threads. You may have to chamfer the BB shell to ensure concentric alignment. These are called thread less or thread through BBs.
Good luck!
Trying to post a video but don’t seem to be able to do that.
The thread on the bottom bracket cup seems worn away, the thread in the frame looks ok.
Will try if I can install a new BB tomorrow.
I sincerely hope the frame is ok, it’s a high end steel frame that is meant to be my bike for years to come…
Oh you may be fine. You can have a shop chase the threads in the BB shell first then try a new BB and see if they can tighten to spec. It probably came loose and you continued to ride until you mashed all the threads away. Lucky it’s an alu cup.
Nice bike btw. 
Good. Way better than BB shell threads being worn away (on the frame itself) which is how I initially read your OP.
Looks like steel frame (and presumably BB shell)? The softer aluminum threads of the BB cup is the sacrificial worn-away in this situation. You’ll need new BB cup(s).
Agree that having a shop check the alignment and chase the threads is a good idea.
Just took a BB from an other bike.
Was able to install this but it was difficult to get it to engage, so indeed probably the threads need alignment. I remember also having this issue when installing the previous BB, so maybe it was not thightened enough.
I just did an 1100 km ultra race with the bike, that probably shredded it.
Just for reference, I’m currently using a self-threading bottom bracket from YST (BB-0901) to avoid any further damage to an aluminum frame with stripped threads on the BB shell. So far so good, but I still feel a lingering pain from discovering that I had bought a frame with a damaged BB shell. 
For the original poster, I would recommend a thorough clean-up and inspection of the BB shell to avoid any further damage there.
As @Manuel_Pata mentioned, I’d clean out the threads of both sides of the BB shell (with a brass wire brush, gently and WD40 to “lift” stubborn debris that might have been worked into the threads) before handing the bike/frame over to the shop to rework the threads, just to err on the side of caution.
It’s not always a given that the shops would do this before they start chasing the threads again. Cleaning the threads yourself also gives you the opportunity to inspect the threads for wear and damage beforehand.
While you’re at it, have a look at the both faces of the BB shell to see if they’re properly faced (and get the shop to do it, if they aren’t) - this is to ensure that the cups of the BB will sit square in the shell, after installation.
