I am sure that they did not have probable cause to search the car, and most likely did not suspect that she had a bomb, but they most likely have regulations that say if you have an expired registration, burned-out head light, et al. This would be in-line with the "broken windows" (or Minneapolis) model of policing. That model understands that police generally catch major criminals through simple infractions like failing to get their car registration renewed, spitting on the sidewalk, or even jay walking.
They could have made her park her car if they wanted to. The registration had expired. I am sure that she would have been complaining even more loudly if they had done that.