Undoubtedly, the loads that stress the blades and head when you raise the collective are induced from the bouncing and thumping the rotors can make under some wind conditions, and not due to the tiny lift forces generated by the blades are they sail aound the mast.
At low rpm, when the pitch is raised, both blades (upwind and downwind) go to the new increased angle, but only the upwind blade sees a lift increase because the other blade is at or beyond stall in its down-wind state, so it is non-aerodynamic. The upwind sweeping blade lifts up more than it would have at flat pitch, and then crashes down as it swings down-wind and stalls and its mate sees the lift. This big up/down swing (we have all seen it, it occurs often enough when the pitch is flat) is vastly increased if you raise the collective, and the shock loads on blades and head are not nice. Additionally, the chance to bump the mast is also increased, as the Bell blurb says.
This crash-bang cycle can be induced if you tilt the swashplate very much with cyclic, too.