Sorry you've been kicked out of Target for this mistake, which reflects a normal risk faced by cart attendants. Sometimes there's only a split second when some car swerves out in front of your carts. Usually a firing at Target is because of blatant dishonesty, stealing money or merchandise, intentionally injuring a co-worker or guest, intentionally damaging company property, or serious absenteeism. If your description is accurate, you did not engage in what's called "willful misconduct", insubordination or theft.
Firing seems to be an excessive punishment, but employment-at-will laws allow this kind of immediate firing.
Most of us who work at Target, myself included, have found that if you are honest about your mistakes Target will give you a second chance. Cart attendant losing control of the carts when a car comes out of nowhere in the parking lot isn't a typical cause for firing. Somebody in your store's management has some other agenda going on.
Sad.