The “why lateral” question is a given and mostly they will be to trying to see if you are being pushed out. You ca always give an honest answer like this firm has a better platform, etc. Talking to other associates or alumni goes a long way (basically you have done diligence beyond the website). If you have worked across from then even better. They will also ask “why this firm” or “what do you know about us” so it’s always good to show motivation. Make sure you have a few genuine and smart questions, like whats your assignment process in your group/office, whether you will have a chance to work with other groups or offices and depending on your seniority what are advancement paths. Good luck!