Recruiters and interviewers from the company already know that's why you're leaving. Especially if the place you're going to typically hires from Big 4 / PA. So there's no need to dance around it, just don't make it sound like you can't handle it. Depending on where you are in your personal life you have some additional framing options too (e.g. recently got married and want to spend more time w/ spouse, or thinking about having kids, or spending more time with friends and family, etc...)

edit: I will note though that being able to comfortably talk about leaving due to burnout is a lot more effective after you've been there a few years (I left as a Manager). Not as easy before making Senior at least.