Probably ate other black holes in it's youth
Yet apparently it's not its current growth, that seems in line with norms, it's its earlier growth that is blowing boffins minds, with earlier growth outstripping anything they had seen before.
But really it probably as a youth swallowed another couple of black holes, Scientists models cannot model what they do not enter into them, if they have no clue of black holes being swallowed then they cannot model it.