In an articulated hub rotor craft, what could cause the blade to flap negatively? For example in forward flight what could cause the blade to flap lower than parallel to the CF at the front of the aircraft ?
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.