Keg gets to heart of this matter. Most incidents of this kind occur when highly professional crews believe that they are following SOPs but, for some reason have their sensory perceptions distorted.
The frequency with which such misperceptions occur is a direct function of conditions in the external environment or the internal mental state of the crew. The probabilities are researched and measureable.
Anyone who leaps to the assumption that this incident was due to incompetence has simply forgotten their human factors stuff. It MAY be the case that the individuals acted incompetently or recklessly, but the probabilities are far greater that it was:
(a) perception distorting factors in the immediate operational environment (as some have suggested; and/or
(b) multiple distractions leading to cognitive overload; and/or
(c) systemic failings in the airline, airport or ATC operating and training procedures (manifesting themselves in (a) or (b) situations).
In my work in the industry (and in other safety critical industries) a good indicator of individual and collective professionalism is the ability and willingness to consider (or wait for others to consider) all these possibilities prior to making judgements about cause.
A significant human factors problem occurs when there is a cultural predisposition in the company or regulatory authorities to default to individual blame. This is because it raises the background stress levels for everybody and therefore increases the frequency with which environmentally triggered misperception (lapses) or incorrect procedures (mistakes) occurs.