I had an interview with a company X with different people, one of them should be my future manager in that company.
I discovered that this person is now in the hiring process in my current company for a position similar to mine, not even for being a manager or something similar.
Now that company X has sent me an offer, but to me, this event is a red flag which made me take the decision to decline that offer.
Since I guess i can't (and I don't want) mention the fact that since my future manager wants to join my current company I don't want to accept the offer,
What would be a good motivation to refuse the offer without being too generic?