The tip-over of scissor lifts in operation has frequently resulted in the death and/or severe injuries of workers. The objective of this study is to enhance the understanding of its major mechanisms and factors influencing scissor lift stability. Both experimental and modeling approaches were used in this study. Two series of experiments were performed under possible tip-over scenarios: curb impact and pothole depression. Based on the dynamic characteristics identified from the experimental results, a lumped-parameter model of the scissor lift was developed. It was applied to investigate the effect of scissor structure flexibility on the tip-over potential of the lift, to understand tip-over mechanisms, and to explore preventive strategies. This study found that the fundamental natural frequencies of the lift were generally in a range of 0.30 - 2.08 Hz, which are likely related to the tip-over. Increasing flexibility of the lift structure generally increased the tip-over potential. The tip-over threshold was also a function of both ground slope and tilt speed of the lift. The results suggest that the lift should not be elevated on largely deformable and/or uneven surfaces such as bridged wood board or a soft soil base. The worker on the lift platform should avoid any large continuous periodic movement or forceful action in the horizontal plane, especially when the lift is fully elevated. Besides the tilt angle of the lift, the tilt speed should be monitored to help prevent the tip-over.