This is called a IRD (Improvised Rhetorical Device). You drop it aside a seemingly reasonable argument so that you can later hijack the entire conversation to

This is called a IRD (Improvised Rhetorical Device). You drop it aside a seemingly reasonable argument so that you can later hijack the entire conversation to talk about what you really wanted to. It's effective in it's primary function but also makes it easy for those on the other side of the argument to paint you as a wild radical.