I believe it is a very common misconception among candidates that they need to explicitly state a hypothesis before laying out their structure. But a much better advice is the following:
Delay the explicit statement of a hypothesis until you have something to ground your hypothesis on! Just stating a hypothesis for the sake of it serves no purpose, since it is nothing more than guessing!
For example, if you are structuring a profitability case, where profits have decreased and you are asked for a diagnostic. Unless the interviewer has given you a hint in the case prompt, it does not make any sense to hypothesisze on the concrete reason before laying out your analysis/diagnostic structure! Instead, you can say:
“I would like to first identify the numerical driver of the problem, which can sit either on the revenue or on the cost side (or both). Based on this initial assessment, I would build a hypothesis on the underlying reasons for the detrimental development, then verify the hypothesis, and subsequently derive measures to address these reasons in order to reverse the trend.”