The answer is – instability of immeasurable states (e.g. those states in a Hydrogen atom that would have energy NOT equal to the energy of the Bohr's energy levels).
It seems to me that many Western physicists think of a wave-function as of an actual real physical field, e.g. like electric field. Having this view naturally makes them wonder what happens to the field if just before a measurement it existed in a huge area of space but right after a measurement it exists only at a specific point. that is why they call it a "collapse" and trying to break their heads to understand what happened. However, a wave-function is not an actual physical field, but just a mathematical abstract - like the Dirac Delta-function. Yes, it does have strange behavior, but what is not strange about quantum world?