A pianist
In any problem with collapsing joints, there can be a medical problem, too.
I have exactly the same problem as described, with the pinky on my left hand. It will collapse, with just the slightest pressure. This is because I have over-extended when playing octaves (I have small hands).
As Alan said, the problem may be higher up. This is correct with my pinky.
The problem is in the finger and ligiments, BUT the control of the problem is at the wrist or higher.
I learned this by putting on a TENS unit, to help a torn ligament heal.
When left alone, my pinky always collapses, no matter what I do--just a stretch the hand will cause it.
However, when this unit was put on at the wrist and higher on the arm, it caused the nerves and ligaments to contract and relax. That muscle and skeletal movement was keenly felt.
Each time the treatment was finished, the nerves and ligaments were a little stronger, as well as more relaxed.
When that happened, it was a miracle--I had complete control over the pinky--no collapse at all.
So, although the problem may be at the pinky, the control of the problem may be higher up. That's something to think about.
Bob T