There are a lot of reasons to cause accidents, but operating factor and equipment factor are the mainly reason:
1) Operating factor
a. Unsuitable lifting way to cause unhooking or swing of heavy goods.
b. Disobeying the operating rules, such as lifting with overload, someone works in dangerous area etc.
c. Wrong command, and inconsistent action etc.
2) Equipment factor:
a. The lifting tools are failure, such as the damage of hook, grab, steel rope and nets to cause the falling of goods.
b. The fault of operating system or safety device to cause accident, for example, the fault of brake device to cause impact and  converging attack.
c. The strength of components is not enough, for example, collapse of tower crane, that's because the overturning moment exceeds the stabilization moment.
d. The damage of electrical equipment to cause the accident.
e. The off-tracking accident of overhead crane , the most reason is rail gnawing.