Oh, I'm sure it's not random. I bet there's a rationale for almost every change in every piece of software UI. These are not bullies making changes just to annoy me; I know that. I can even understand why they stopped supporting the old look & feel after a grace period. I'm a software dev too, after all.
But from the point of view of a random user (me), it seems bizarre. The Gmail team changed things for no apparent gain. I don't care about Material Design -- I don't even know what that is! All I know is that the immediately previous incarnation of Gmail looked less cluttered and felt snappier. I know it's not accurate, but sometimes it feels they make UI changes just to show the app is not abandoned, whether this changes make sense or not.