I would, personally, discount anyone who starts stating a hypothesis right after drawing out a structure. You know very little about the case, you have asked no clarifying questions, you understand just the top surface of the case yet, so you are basically stating a hypothesis that is grounded in guess-work. Here's the way I typically reach a hypothesis.
1. Once you are handed the case, go through it (some interviewers read it out, then hand data, some others just want you to read it yourself -- MBBs typically "give" the case).
2. Ask a few questions about the case. For example, if it's a falling revenue case, then at least try to narrow it down to (a) is it industry wide / economic downturn (b) are we in a seasonal industry. You can deep dive into whether we are the only one suffering, or whether only the smaller players are suffering due to an amazon-like behemoth eating their lunches later in the case.
3. Narrow down your "deep dive" areas to 1-2 places. For a revenue-loss case, you might want to focus on product quality or change in demographics or new entrants. For a profitability loss case, you might narrow it down to rising costs, inefficiencies in capital allocation (e.g., bad forex hedging, or so on), or pricing pressures.
4. Only then state a hypothesis. Something like "My hypothesis is that this is either an issue of pricing or increasing costs. I'd like to further narrow it down and to do so I'd like to look at the financials around revenues and costs."
5. Once you get the data, have reviewed, then fine-tune the hypothesis and you could say something like, "having gone through, I'd like to hypothesis that this is most likely a cost issue; I'd like to understand whether this is only us, or everyone..." etc etc..