Rotor wash
LTP90 is very correct about the rotor wash. Years ago I was landing a C172 at the local strip and one of these Skycranes was adjacent to the runway near the approach end. It was running with the rotor disk turning but firmly on the ground. Even with the blades at fine or flat pitch (ie not pulling pitch) there was enough rotor wash that in the flare as I was landing I thought for a split second that I was going to cartwheel and crash as the right wing flipped up about 20 degrees. It felt like 50-60 so I'm guessing at 20 but it was a serious gust when not at all expected.