Cold-welding in a vacuum environment would be my guess as the cause of the sticky bolt. Keep in mind that satellites employing optical instruments are very sensitive to deposition of outgassing vapors from materials used in the fabrication of the satellite. As a result the rules are very strict as to what materials can be used. This rules out Loc-tite, for instance, as a way of keeping bolts from loosening. Lock washers are out also as they tend to generate fine metal shavings which are also undesirable. Nylock, as I recall, is also banned because of either vapors or particles from the Nylon. This leaves mechanical interferance as the preferred way to secure fasteners. (It’s not the vibration in space one worries about; it’s the vibration encountered getting there.) When metals are forced together in a vacuum, there is a tendency for the metals to stick together, a form of cold-welding. This has been a problem in the past with satellites such as the OSO (Orbiting Solar Observatory), which had a rotating body with a fixed solar-cell array which kept the observatories instruments pointed toward the sun. The bearings had a tendency to stick. It is possible that NASA has forgotten this early experience; witness the problems with the bearing failures on the current space station.