A theory:
You said it was slow - there is nothing more troublesome to a ride like Test Track than a lack of a line.
Without guests, vehicles aren't dispatched post-haste, though the CM's know they should be regardless. The CMs start to slow down a little, you would too if your day was finally getting easier. However, this is bad for the ride. The slower the dispatch of vehicles, the closer and closer you're getting to a cascade stop.
A cascade stop is caused because the vehicles reach a point where forward motion isn't possible because too many vehicles have ended the ride but not unloaded. Essentially, it's a backup. All the spots are filled and there's nowhere to go, so everybody stops - a safety measure.
To avoid it - you have to keep the vehicles moving in the station wether or not there's people to ride. That's why they often let people re-ride, as it's faster to refasten the belts than to unload. On a boat ride you can float around slowly in a scene bumping boats, but rides like TT don't have that option.
When a cascade stop happens, there's usually an unceremonius "thunk" and everything stopping abruptly. If the vehicles in the loading area were jogging forward to their next load/unload positions it'd be a pretty loud and unfriendly thunk as they all slammed on their brakes at once.
Just a theory.
