Nineveh and Nimrud: why Now?

I have a few thoughts about the latest reports of heritage destruction in Iraq. Let me emphasize that what follows is personal speculation.

In rapid succession we have had the videos of the destruction in the Mosul Museum and the Nergal Gate at Nineveh, and now, reports that the Islamic State has bulldozed the ruins of Nimrud. (Although, as  Conflict Antiquities notes, there's no confirmation from ISIS, ; everyone is citing an Iraqi government claim, as they also note, if it hasn't happened yet, it's probably only a matter of time. And UNESCO Director General Irina Bokova credits the report and has denounced it as "a war crime.")

Most analyses I've seen emphasize the Islamic State's view of "idols" at least those too large to sell on the antiquities black market) and their apocalyptic  worldview. One question I haven't seen addressed is, why now? ISIS occupied Mosul in June of 2014. Even if the videos of the destruction are several weeks old, as some experts suggest, why didn't they do it last year?

I know, maybe their first priority was ethnic cleansing of Shi‘ites, Christians, and Yazidis. But I'm wondering (perhaps wishful thinking is involved) if they're unsure of their long-term hold on Mosul and Nineveh Province?

I realize ISIS doesn't exactly display a sense of insecurity, but they have been facing setbacks from the peshmerga to the north, threatening their logistical lines to Syria, and now the Iraqi Army's push on Tikrit is starting to look like a serious campaign that might gain some traction.

I don't want to overstate the possibility. The Iraqis haven't yet taken Tikrit; even if they do, it's over 200 kilometers to Mosul through all-Sunni territory and every step will be contested.

But I still think the question is worth asking.