A mathematician called Bernoulli found that areas where fast moving fluids are present have a lower pressure than the surrounding environment. Furthermore, he found that water running over a curved surface of also exhibits the same low pressure, the faster the water the lower the pressure.
So, why did the ball move toward the water stream? Fluids (and air) want to move from high pressure areas into low pressure areas. You created a situation where the wet side of the ball had lower pressure compared to the dry side of the ball. The high air pressure on the dry side of the ball pushed the ball into the low pressure water stream.
