I gather the control surfaces on the 787 are operated by electro hydrostatic actuators (EHA), probably at least two per control surface for redundancy. For an EHA to work it needs an electrical supply plus a pulse code modulation (PCM) signal provided along the power supply wiring. Therefore, a reasonably high capacity electrical supply is vital and must always be available to power the EHAs to enable the cockpit controls to operate control surfaces, raise/lower the landing gear, etc.
Given the above, it is understandable that any doubts about the reliabilty and fail-safe functions of the 787's electrical system is going to make the FAA 'twitchy'. I suspect their review will be re- checking that there is no weakness in the design whereby a single point of failure could 'knock out' all the built-in redundancy.