Answer: Wing flex on take-off
When GlobalFlyer took off it did so with a substantial amount of fuel onboard, both in the wing tanks and in the boom tanks. When Steve lifted off the wings flexed upward because the wing fuel was supported by the lift generated by the wings. The boom tanks do not create any lift to sustain flight and resisted that motion upward, due to gravity acting on the weight of the fuel in the tanks. This results in the situation you see on the takeoff video where it appears the wings wanted to lift off prior to the GlobalFlyer.
Consider this simple model: Holding a yard stick at both ends, suspend a bowling ball from the center using a rope. The act of you holding each end is the "Lift" during rotation. The act of the bowling ball is the immense weight contained in the boom tanks being pulled by gravity, resisting the takeoff.
One factor that led to the large "Flex" in the wings relates to the composite material that Scaled Composites constructed the GlobalFlyer with. It allows a larger amount of flexibility than Aluminum construction would.
Another factor is the extreme length of the wings. The longer a wing is, the more susceptible it is to wing flex from wing loading, as well as turbulence.
A video of takeoff can be found here.