There has been a general decline in interest in Xbox I think. Many people have migrated away to PS4 and others are simply choosing PS4 over Xbox one when they buy a console. Lack of exclusives, weaker versions of games, smaller range of games etc.
I don't think its related to Switch at all its more related to their failure to compete with ps4.
I own both ps4 and xbox one and as much as love my xbox it just feels like the success of ps4 is accelerating. It's not really a choice between xbox one and Switch they don't have the same type of content. Multiplayer first person shooters are really important on PS4 and Xbox but there is nothing like that on Switch and that is just one example of game genres that are popular.
The Switch is it's own distinct area with Nintendo developed titles and portability. It's a good console to own in addition to owning a PS4 or Xbox but not really an alternative to one of them.