45° rotation make sense when you want to avoid having half of your traces exit a dense chip towards the board edges, assuming (which often is the case) a rectangular board.
I guess in this particular case, the (many, and sensitive to total length and relative length with respect to each other) traces connecting CPU and RAM had been routed in isolation first. These chips seem to be 45° rotates against each other.
Then the group of CPU & RAM had been rotated too leave enough space for power/camera/HDMI, I'd guess.
It looks strange, but frankly there is no real reason (besides aesthetics) to favor any particular orientation.
I guess in this particular case, the (many, and sensitive to total length and relative length with respect to each other) traces connecting CPU and RAM had been routed in isolation first. These chips seem to be 45° rotates against each other.
Then the group of CPU & RAM had been rotated too leave enough space for power/camera/HDMI, I'd guess.
It looks strange, but frankly there is no real reason (besides aesthetics) to favor any particular orientation.