The static build up on the flaps is allowed by the fact that the flaps as well as the slats are free floating relative to the airframe. This problem can lead to serious concequences if a partially deployed slat were to be struck by lightning. By serious I mean that the wing at the least could catch fire and at the worst, the outer panel of the wing could be blown off. This problem has nothing to do with an uncommanded deployment of the flaps and slats. You are correct about the solenoids that control speed and direction but in this case it is an internal leak that bypasses the solenoids and goes directly to the hydraulic motor on the effected hydraulic module on the PCU. It is true, the PPUs would indicate movement but if the CSU is in the flaps and slats retracted position the computer is out of the loop and can't stop the action. This was demonstrated on the slat Iron Bird during testing. The manufacturer of the PCU was supposed to notify Airbus about the runaway but they didn't. Instead they reworked 17 ship sets and contacted Lufthansa and Swiss Air telling them that they had improved the design and would swap out the reworked PCUs for the PCUs on the respective aircraft. This was in total conflict with the contract and it reflected poor judgement on the part of the company officers. After I notified the FAA the Vice President and Cheif program manager were fired but the lack of bonding and the design of the PCUs were never changed. I have contacted many US operators of Airbus aircraft telling them of the potential for runaway and that they should incorporate procedures in their training syllabus in how to counter the runaway. After about a year in doing this I was contacted by US Airbus telling me that I was defaming their product. I told them I was only interested in safety. They told me to stop and I told them that I would if I could talk to one of their engineers. They told me they would set it up. That was over five years ago and I am still waiting for the call. That is why I posted my material on this forum. If you are interested in helicopter safety please read my postings on ROTORHEADS IN THIS FORUM.
More recently I flew in an Air Canada A320 from Toronto to Chicago.. I was up front on the flight deck telling the pilots about the potential for runaway on the flaps and slats due to the hydraulic system architecture which is the same on all Airbus aircraft. While I was still talking, the First Officer who was also Air Canadas' director of flight safety, handed me a fax. It indicated that two days earlier an Air Canada A320 had an uncommanded retraction of the flaps on take off. In this case, the CSU was in the flap slat takeoff position and the computer didn't catch it. The pilots almost lost the aircraft and their only salvation was to apply full throttle and fight the aircraft until it was in a stable flight regime.
A similar internal leak caused the thrust reverser to deploy in flight on a Lauda Air 767 causing it to crash and it may be eventually proved that an internal leak caused the 737 rudder problems.
That is my opinion.