Well with the booming success of the Facebook phone, you can't blame them for feeling a little shaky. Though I do think video on Instagram is a natural progression.
I guess these are both valid points.
But also the trend of having a scrolling feed, and a social side to the app as well as hashtags, that seems to be the standard now to a social media app or network. IG has taken it to a step where their "idea" has sparked interest from their followers who haven't seen it before anywhere and also hatred from vine users since it is such a mainstream app.