Capacitive loads affect frequency. Crystal oscillators must be matched to varying loads which account for the wide variety of crystal components available.
Maximum drive power must also be considered to prevent over-driving. If a quartz crystal is constantly exposed to overdriven conditions in excess of the anticipated maximum power, it can age quickly.
Even placement on the board can affect capacitive load, whether it's relocating the crystal itself or rerouting of other components. All of these are potential sources of mechanical resonance which have to be accounted for and tested in the oscillator circuit design process.
Alteration of the power supply load can cause a change in the effective resistance of the oscillator circuit leading to frequency drift. One solution is to use a regulated power supply, ensuring that the output voltage will always stay at the rated value of the power supply, regardless of the current that the device is consuming.
That would seem to say that any and all changes on the PC board due to degradation of other components, corrosion or heat can actually cause oscillator drift.
