Is it dark when you are driving home - if it is the different lights may frighten her. Maybe putting a light rug over the straps of the capsule, or trying to find a hood for the capsule may help.
if it's daytime is the sun on her face, although usually they close their eyes with this and will go to sleep. Can you play some soothing music, maybe that will help (even play in the morning so she understands that is her travel music (or try something really noisy with lots of beat).