Why did the dog in pavlov's famous experiment salivate when it heard the sound of a tuning fork even when no food was present?

The dog had been conditioned to salivate because food had been presented so many consecutive times in the past that it had learned to associated the tuning fork with the presentation of the food. When the fork was sounded even without bringing food out, the dog still is under the assumption that the two stimuli (fork and the food) will be paired, and subsequently began to salivate.