I want to transit from one Big4 to another - how to explain it on interview?

Hello everybody,

I'm currently working in Big4 and for some reason want to quit my firm and transit to another Big4 - to same department, same grade, same city. Obviously on the interview I'll be asked why.
Don't want to disclose the true reason, so I will have to bullshit through it.

If i say I didn't like the people or want the bigger salary etc. all these in some way or another will portray me negatively. What are the possible plausible reasons for changing one Big4 firm to another, to just the same role, that will not show me in bad light?

Thanks in advance!