Helibiz,
Whoever did your sling endorsment should have covered this. It's a common occurance in sling / winch op's to get static buildup on the line. This static charge will "go to ground" at the first opportunity (ie: through the loadmaster / rescue crewman) if the hook is not grounded before they touch it. Normally, allowing the hook to briefly touch the ground first will cause sufficient discharge, also a well insulated person is less likely to recieve a shock. Dry & dusty environmental conditions will normally make static buildup more likely too.