Final answer:
Radley thought the School Master would kidnap Agatha because he overheard the School Master discussing plans for a surprise birthday party for her.
Step-by-step explanation:
Radley misunderstood the School Master's intentions when he overheard him discussing plans for Agatha's surprise birthday party. As the School Master schemed the surprise, Radley mistakenly interpreted the conversation as a plot to kidnap Agatha. The confusion arose from Radley's partial hearing of the conversation and a lack of context. His mind pieced together fragments of dialogue, constructing a misguided narrative that led him to believe in the School Master's ill intentions.
In reality, the School Master's intentions were entirely benign, focused on organizing a celebration to honor Agatha's birthday. Radley's misunderstanding underscores the significance of context and complete information in interpreting situations accurately. Misinterpretations often arise from incomplete information and assumptions. Radley's misjudgment serves as a reminder of the pitfalls of drawing conclusions based on partial or misheard information.
The correct reason for Radley's belief is a misunderstanding of the School Master's plans for a surprise birthday party for Agatha.