That might be why then. They probably didn't see enough of a history between you two, and just wanted to talk to your spouse to make sure that it is legit. It sucks though that it will delay the application more.
Just wanted to follow up on this.
My husband and I have been together since 2016 and were married in Feb of 2018 and we just finished our interview last week and the immigration agent approved our interview on the spot.
It actually seems as if more people have been called for interviews recently, though I am not sure of the reasoning for this.
Our application has been in process since May of 2018 so we're getting quite frustrated with how long this is taking. Seeing June applicants receiving their DM and PPR only makes it worse and goes to show that there is no proper formula for how any of this works.