As a former employee, my best guess is that you were right- the cashier was probably off at 5 or perhaps had a break scheduled. I imagine that he either needed to go (perhaps he had a ride waiting, had to make a bus, had to pick up a child from child care, etc) or was trying to leave on time to avoid overtime (big box retailers are serious in their attempts to not pay overtime to employees and will blame employees for not leaving on time).

When an employee is scheduled to a specific time (5 pm), when they are off, they are entitled to leave. Just like when a store closes at a specific time, they can (and sometimes will) kick customers out. They should not be expected to check out additional customers because the store is busy without being asked.

That being said, that is why there are managers at the front lanes of Target. Their jobs are to "manage" the cashiers, ensuring that they get their breaks and are able to leave on time. He or she should have redirected to you to a different line, stepped onto the lane to check you out, or turned the cashier's light off well ahead of time, enabling the employee to leave on time and take care of his line.

I believe you find fault with the cashier who has a right to a break or to leave their shift. It is great feedback for the management of the store, who hopefully can avoid a situation like this in the future.