I am pretty sure they won't be asking you this question. What they could ask, on the other hand, is "why do you want to switch to McKinsey now from your current firm?" For this, you can formulate any reasonable answer around your reasons for switching - e.g. work with greater variety of clients, higher impact projects, training, etc.
I would also say that for McKinsey specifically, the "FIT" part of the interview is very structured as part of the Personal Experience Interview (PEI). Therefore, you are less likely to get these types of "behavioural" questions than at BCG/Bain, and likely will just get one PEI Question (details of PEI have been discussed at length in previous Q&As).
I am pretty sure they won't be asking you this question. What they could ask, on the other hand, is "why do you want to switch to McKinsey now from your current firm?" For this, you can formulate any reasonable answer around your reasons for switching - e.g. work with greater variety of clients, higher impact projects, training, etc.
I would also say that for McKinsey specifically, the "FIT" part of the interview is very structured as part of the Personal Experience Interview (PEI). Therefore, you are less likely to get these types of "behavioural" questions than at BCG/Bain, and likely will just get one PEI Question (details of PEI have been discussed at length in previous Q&As).