Human error
Maintaining a modern aircraft is a very complex task, it has to be constantly reviewed for updated procedures that arrive from manufactures, sub contractors and regulatory Authorites.
It is inevitable that human error can creep in and occasionally things get missed, if an airline misses an item it is far better they come clean and fix the problem than cover up the mistake, that way the error in the system gets exposed and procedures amended to correct the error and you can bet that publicity results in the six other airlines that have made the same error get to know they have made an error and so correct it.
Rather than slagging off SWA for making the error they should be congratulated for coming clean, and stopping the aircraft from flying to correct the issue.