I've noticed today that I've had a lot of balls start spinning to the side (like a curveball would) after I throw them straight at the Pokemon. I'm not spinning the ball beforehand to get the curveball bonus, and it doesn't appear to happen if I throw the ball at an angle to compensate. It also seems to happen more if I'm throwing the ball further.
Is there a known mechanic or any theories concerning this?
Update: I spent some more time trying this out and talked to some co-workers who play the game. We also noticed that some balls thrown intentionally as curveballs will sometimes spin the opposite direction or go straight in the direction that they were thrown.