Here's what we know about the incident so far. Qantas flight 32 is the morning departure from Singapore to Sydney. Very shortly after takeoff, one of the engines suffered what appears to be an uncontained failure (more on that in a second). The airplane diverted and everyone was fine, but now Qantas has grounded its entire A380 fleet until it figures out what went wrong.
An engine failure in itself isn't usually a major issue. The A380 has four engines, but even twin-engine aircraft are designed to be able to fly safely to an alternate airport on a single engine. In this case, the A380 had three engines left. No big deal, right?
It's more serious in that it was an uncontained engine failure. A contained failure means that no parts of the engine left the engine casing. But this was an uncontained failure with debris falling to the ground and potentially damaging the airplane along the way. That makes it far more serious.
There have been catastrophic uncontained failures in the past. For example, you remember the 1989 crash in which United 232 did cartwheels in the Iowa cornfields after a blown engine severed all hydraulic lines. After that incident, the lines were redirected so that they didn't all go through the same point on the airplane.
But nothing catastrophic happened here with Qantas 32 -- the plane landed safely. There have, however, been concerns about the engines, the Rolls-Royce Trent 900, in the past. As noted in the Things with Wings blog, authorities have issued directives on these engines for the possibility of an uncontained failure. If those changes haven't been made by Qantas yet, then it might be a good reason to ground the fleet.
My guess is that there's a lot more going on behind the scenes that we don't know about. Maybe Qantas has seen some issues around the engines before and this was the last straw. Maybe there was greater damage to the airplane than is being released to the public at this point. There are a lot of ways to speculate here, but in general, having an engine failure after three years in service is not enough to instantly ground the fleet.
The cost to Qantas for doing something like this is enormous. There are now hundreds of people per flight being stranded around the world until alternate transportation can be arranged. And with the A380 fleet sitting idle, it's not going to be easy to clear the backlog. Those people will have to put up in hotels, given food, and potentially put on other airlines. Though I'm sure Qantas will try to recover these costs from the manufacturer, it's still a major step to take. That's why there has to be more to this story.