Why can an instrument go out of tune during one rehearsal even when nothing is broken?
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Answer: Temperature, moisture and playing can change how the instrument vibrates
Temperature, moisture and playing can change how the instrument vibrates ✓ — Conditions can change during a rehearsal, shifting the instrument's vibration and pitch without breaking it. Strings can change tension as they warm or stretch, and moisture can change wooden parts. In wind instruments, warmer air increases the speed of sound and can raise pitch; that is not just the instrument expanding.
The instrument must have aged by years for its pitch to change — Years of aging are not required for pitch to shift. Wear can affect long-term tuning stability, but temperature and playing can change an instrument within one rehearsal. That is why musicians keep listening and adjusting after the initial tuning.
Sound waves scrape away enough material to change the notes — Ordinary playing does not scrape material away with sound waves to retune the instrument. The relevant changes include string tension, dimensions of parts and the speed of sound in the air inside wind instruments. These can shift pitch without removing material.