Good speculation. I bet it is 3. I don't think it could be 2. After all, Liam and Andy make separate videos, so Andy never had to be exposed to anything he didn't want to do. Anyway, Andy has always accepted that Liam is wild: Andy told me years ago that Liam "will do anything" and he wasn't just referring to girls!
My guess is that Andy has found it difficult to adjust to the fact that his channel introduced and monetised all these boys who now make lots of money on their own account. Perhaps some of them pay him a share or agree to create content together but that Liam doesn't want to work to Andy's schedule or pay him any due. Perhaps Andy has seen his revenues decrease as Liam's has increased.
They've been friends for many, many years. It would take something like money to break up that kind of friendship.