The sparks produced during sharpening of a knife against a grinding wheel - they leave tangentially at the rim of the wheel.
It can be attributed to the phenomenon of Inertia - of 1st law of motion of Newton.
When sharpening is done, very tiny pieces on the surface of the grinding wheel get broken from the wheel. These small stone pieces have a linear velocity in the tangential direction (equal to radius * angular velocity) just before breaking off from the stone.
After breaking off, they fly off in the tangential direction, as they continue to move in the state of uniform motion with a constant velocity. Then gravity comes into picture, and pulls them slowly downwards. However, we see when the sparks are red hot, after a very small time duration, these particles cool off, and so we do not see them clearly as they fall down.