Solder balls forming at the side of components and not at the joint surface are referred to as solder beads. This is simply to avoid the confusion between the many other solder ball phenomena.

During the assembly operation solder paste gets under the body of the component. As the board passes through the reflow oven and into the reflow zone the paste turns into a liquid. All the solder balls coalesce together to form a solder joint. The same thing occurs to paste under the part in this case the increase in size of the solder lifts the part to allow the liquid to escape. The component lowers back on to the board leaving the solder bead.

To determine when the paste enters under the parts check paste printing quality. Remove parts prior to reflow and check for paste. Pass a fully loaded board through reflow changing the final zone temperature to prevent paste reflow. When the board has exited check for paste under parts.
By finding out when the paste gets under the parts it is possible to eliminate the problem.