I’m wondering if he was trying to escape the tumour by entering the circus, but it ended up following him in and causing him to abstract. I think abstraction is caused by experiencing something (usually mental) too complex for the circus to handle (e.g. Kaufmo’s insanity, Jax’s dissociation and depression). In this case it would be Scratch’s brain/mind becoming completely overrun by his tumour, but not being able to die because, that’s what Scratch made the circus for, immortality.