I doubt we'll ever know for certain, though Ms. Cooper could conceivably start blogging again (she stopped just before joining the WMF) and drop a hint or two. If I had to guess, I'd say it was her idea, not theirs - she probably got frustrated with being, for all intents and purposes, a glorified bandwidth optimizer and patch installer (applyer?), which wouldn't have been what she signed up for.
What she didn't stop doing (after being hired as the WMF CTO) was tweeting. At least half of
her Twitter feed is about bandwidth and server problems being fixed, another 30 percent is her announcing her arrival at various airports, and the rest is just links to various OSS articles that (mostly) have little relevance to WP.
It was broadly hinted at here on WR 70 weeks ago (by K. Martin, myself, and others) that if her job was mostly going to consist of sticking band-aids on wounded servers, she wasn't a good choice for it and she wasn't going to be happy doing it. I'd say the victim here, if there is one, is probably Cooper herself. I wouldn't be the least bit surprised to find out that her job interview with the WMF board consisted of them reassuring her that she wasn't going to have to do any of the down-and-dirty stuff, and that she'd instead be concentrating on "high-level strategic technology planning" and various other things that the WMF has never cared about one iota.
If they were realistic about the nature of their organization, the WMF would give the CTO job to Brion Vibber and give him a nice big raise along with it. But they never have been, and in any event, Erik Moeller probably wouldn't stand for it.