I've encountered that too with all the stock boxes.
Thats because the
physics friction is set quite high to allow the boxes to be pushed across the floor, which at the same time makes the player kind of "slid" on top of the box so he falls down immideatly (or the box gets pushed away under him).
Unfortunately changing value for
physics friction doesn't actually change the box's behaviour even if set to zero.
It's a bug and honestly I think the idea of "get box - stack box - jump on box" is SO old, new FPSC games should be able to do without.
In case you find my grammar and spelling weird ---> native German speaker ^^