My understanding is that if the bill were to lead to more complete databases, by which I mean both the U.S. and Canadian governments would have a complete picture of a traveller's entries and exits over time, that could certainly paint a very detailed picture of a person's life. That information existing for a lengthy period of time can pose privacy risks, both because of the risk of the information getting out—being stolen—and because it could be shared with other government agencies for whatever purpose it may be.
On October 24th, 2017. See this statement in context.