according to me schedule variance can occur due multiple factor especially those factor which affect the causes your project to lack behind in terms of progress and this factors are:
Poor baseline schedule (does it reflect reality?)
Subcontractor/vendor cannot deliver when needed
More/less effort than planned (be specific)
Insufficient resources (staffing)
Labor disputes/work stoppage,
Resource availability (is it there when I need it?)
Requirement changes
Apart from this on of the major factor affecting is planning if things are properly planned first hand than schedule variance can be positive which means ahead of schedule or at least be on schedule.