I can't speak for everyday at Test Track, but it just so happens I was doing data collection there on the 4th. What happened that day was that Test Track shut down for 40 minutes due to an electrical storm in the area. The majority of people in the Fastpass line opted to leave and were given new passes that would allow them to return later in the day. So when Test Track opened back up, the Fastpass line was overrun by all the people who would normally have used their passes during that 40 minute window. For the rest of the day the cast members were doing their best to burn through the Fastpass line, but that put the Standby line at a virtual standstill. (I was only monitoring the front of the line, but from what I understand, Standby hit a 2 hour wait time that day.)
Now the rest of what I have to say is just speculation on my part. I think there are two major problems that cause such horrible wait times at Test Track. The first is that the attraction has such slow load times. There are six people per car and, I believe, four load stations. So you only get 24 people in each load cycle. If anything happens to slow that down (Grandma forgets to buckle her seatbelt or the previous riders forget that they have to get out of the car) it has a ripple effect on the whole line.
If the line backs up even a little, then the people can't get out of the briefing room. If there is even one person in front of the exit to the briefing room, the doors can't shut and this means they can't open the doors on the other side to start the next preshow. Therefore, the line stops.
The other problem is Fastpass. Without Fastpass there would be just one line that, while slow, would at least have steady progress toward the ride. (I'm ignoring the Singles line here since it doesn't really play into the discussion.) The wait time would quite probably be more accurate as well. With Test Track's frequent shut downs (due to weather or mechanical problems) you wouldn't have an unexpected number of Fastpass returns. And even it the line did reach 2 hour wait times, I think people tolerate that better if they are actually steadly moving forward as opposed to standing the the same spot for 30 to 40 minutes at a time.
That's my take on things.