Usually this type of behavior is a far deeper issue. For many there are unseen underlying emotional issues that are carried out in various ways, nagging is one, I could go into a bakers dozen of others but I won't. I will however give an example....
A lady has issues with her SO, no longer loves him, wishes he would leave but she does not have the strength to leave herself for reasons of self guilt so she attempts to shuffle the blame off to him, accusing him of things that in reality he never has done, Constant nagging, bringing up heated issues to get a fight going, all in the hope that he gets so tired of the mental and verbal assaults, he will leave her. She wanted the relationship to cease so she makes him make the first move. Eventually he does leave and she gets her wish on "his dime"!
This is just a more extreme example, but the fact is, it was not that she was a nagging wife or nitpick, just used that as a tool to gain a desired result.
OK now you that have nagging better halves, this does not mean they are doing the same thing......some do just like to complain!