That one is fairly easy to answer.

You only need two particles to collide but not coalesce and the chances are that there will then be a residual rotation in each particle.  In a large cloud of gas and particles it is almost certain that some will be rotating and the chance that those rotations cancel each out is very unlikely.  

As they are attracted to each other to form stars then some of that rotation will cause the growing body to rotate in a particular direction - as the object gets denser and denser and shrinks the speed of rotation will increase in accordance with the law of Conservation of Angular Momentum, (just like an ice skater pulling in their arms makes them spin faster).  So you only need a very small rotation when the 'cloud' has a very large diameter to end up with a noticeable rotation when it shrinks to the size of a star.