Eagerly awaiting Sean M's take on this.
People might be looking at this from the wrong angle though. This may not be AUH-instigated as much as a SHJ initiative (as in, benefits Arabia just as much if not more).
There are actually 4 major aiports (AUH, DWC, DXB, SHJ) in a belt which takes about 1.5 hour by car (approx) from AUH to SHJ (without traffic) + another one (Bateen) which is smaller and located very close to AUH.
The utilisation rates of these airports are all over the place however. AUH (with Midfield) is multiple times the size it needs to be. DWC is frozen as of now (and while it's 10 times smaller than the initial masterplan, still has a lot of headroom in its current form). The other two though (DXB and SHJ) are bursting at their seams. SHJ is currently undergoing an expansion and DXB periodically re-arranges the deck chairs to make room.
Utilising the spare capacity at AUH and DWC and alleviating strain at DXB and SHJ has long been the goal. With SHJ now at capacity, Air Arabia needed to find space (which it absolutely cannot next door at DXB). AUH seems like the logical choice for their secondary hub. Etihad will most likely retreat from low yield short haul routes and cede them to Arabia (and continue to lower their costs on the higher yielding longer strategic routes). Same strategy as Emirates + FlyDubai. Air Arabia is infact looking for planes right now and I wouldn't be surprised if they simply acquired Etihad's narrowbody fleet.
Meanwhile Arabia can use AUH as the new connecting hub and use SHJ more for point to point traffic (for passengers originating or disembarking in Dubai or Sharjah).
AUH tourism growth is still pretty healthy and hasn't levelled off as much as in Dubai. If and when Visa on arrival is implemented for India, it will bring a deluge of business to all routes in and out of UAE and is a low hanging fruit they haven't picked yet. So there is a lot of potential growth in traffic that needs to go somewhere (and absolutely can't be accomodated in DXB or SHJ). AUH especially has shown strong tourism growth in recent years and Etihad may not be willing or able to bring in lower yielding pax from tier 2 destinations. So it's strategically important for AUH that someone else can.
Also, recently Wizz Air announced direct DXB flights starting this month. This opens up Europe daily from as low as 100 USD *Return* from UAE and London from 250 USD return. These aren't promo fares and is a game changer for the market. As a long thin route operated by a European LCC, it would be interesting to watch if this will be sustainable in the long term when no such route has been in the past (though I suspect it will be a success, even if they double the prices!).