Why does my in-wheel BLDC motor have sinusoidal back emf (line to line) when rotated by external system?
You are measuring the line-line voltage and not the phase voltage.

The characteristic "trapezoidal" backEMF is only present at the phase voltage. When you view this line-line the waveform appears like a pointy sinewave. Then there are specifics of the build: magnet span, width of stator tooth foot... all these things influence the shape of the backEMF.  With a high polecount the arc needed for a magnetic gap will be very small and probably non-existent. 



And thus the line-line at the terminals would appear:



in practice you won't see this and it will naturally be rounder as this is the mathematical concise result. The point still stands. Line-Line is more sinus than most appreciate.