The reason wind drift isn't linear is because the speed of the wind is being added to the bullet, sideways. If the wind hit the bullet at 25mph and then completely stopped, the bullet would still be sliding sideways under the momentum imparted by the wind.
Basic physics. An object in motion tends to stay in motion.
The bullet begins to pick up its own horizontal speed.
If the bullet wasn't picking up horizontal speed of its own, drift would be linear.
But the bullet is picking up speed, it's accelerating. That's why the path is curved.