Due to the PMI website definition, schedule variance (SV) is the difference between the earned value (EV) and the planned value (PV). [SV = EV-PV].
There are several factors that could affect the schedule variance negatively in any project. It mainly depends on the ability of the project manager and the team to estimate the achievement level and the overall performance accurately. Examples could include a sudden urgent leave of the project manager for a period of time, and this incident would certainly have a big impact on the process flow and on the overall plan. Another factor could be the change in the market demand during the project period which leads to the need of making big changes on the project goals, specifications and metrics to meet the user need at the end.