Why superconducting qubits need periodic calibration?

Question

I want to understand in some detail why superconducting qubits need periodic calibration. The usual, hand wavy explanation is environmental effects that tend to vary. However, I suspect that the actual picture is more complex than that. I want to understand in particular which effects are purely external, like the earth magnetic field (in lack of a better example) and which are internal to the superconducting device. I am especially interested in those factors that are internal to the qubit itself, like manufacturing defects and physical characteristics that may change in time. A good reference on this subject would be appreciated.