The reason it is it is important to look at each step is because that is how we learn safe administration. Step by step, check by check. It's not at all like a car wreck where everything happens in an instant.
If she had done just ONE safety check, she would not have given the patient vecuronium.
And no. You don't go from being a prudent nurse who has occasionally skipped one safety check due to being overworked or overwhelmed or having momentary inattentiveness to one day skipping Every. Single. One.
More likely in her effort to be fast and efficient, she got into a habit of skipping those mundane/repetitive steps. It all caught up with her in the worst possible way, highlighting to everyone why those every single one is so important.