The fact that the appearance of an irrelevant distractor, while an individual is recalling a word list, is evidence of the interference theory of forgetting.
This theory states that forgetting is caused by the appearance of competing memories. There is 2 categories. Retroactive interference occurs when new information interferes with the retrieval of old information and proactive interference occurs when old information interferes with the retrieval of new information.
In this example, this is a case of retroactive interference because an irrelevant distractor (new information) appears while the individual is recalling items on a word list (old information).