In an articulated hub rotor craft, what could cause the blade to flap negatively?
The first thing that comes to mind does not occur in forward flight. That would be a hard landing. Depending on the rotor design and the amount of articulation and the landing gear type, a hard landing can result in some serious problems, notably, Ground Resonance.

In-flight, it may be possible to achieve with a rapid unloading of the rotor (Zero - G) plus cyclic input. One adverse condition which causes negative flapping is Mast Bumping. Usually, if not always fatal. Mast bumping takes a specific set of conditions and control inputs, to take place. It is typically occurs with 2 bladed teetering rotor systems at zero G.