The very well written biography of Marissa Mayer that recently appeared in Business Insider was very illuminating about the current ongoings at Yahoo. Marissa appears to be a very data driven person, always looking for "proof" of display/design feature ideas and concepts, even for whether a border should be 3, 4 or 5 pixels wide.
Additionally, she had made a last minute change to the color scheme of the recently revamped Yahoo Mail which necessitated significant man hours at the 11th hour to implement and was detrimental to team morale and cohesion that had been painstakingly developed since her arrival.
I'm sure moving forward there will be more challenges like this that Yahoo will face. It will be interesting to assess whether they are due to the vestiges of incompetency at Yahoo as she believes, or due to her failings as a leader, because let's face it, according to the profile, this type of a UI design change would have had her hands all over it and would've needed final sign-off by her.
UI Design changes are by their very inherent nature controversial, people like things the way they're used to them. Marissa's approach was already problematic at Google, it had problems scaling as the company grew in size, but at least there were people there to manage and mitigate her. There's no-one at Yahoo like that. She is a very authoritative leader.
Disclaimer: I don't know her personally nor have I ever met her or met anyone who has met her. My impressions are all based on profiles of her like the one linked above (which I am not affiliated with but simply found interesting)