Oklahoma suffered a 5.6 magnitude quake on 11/5/2011 which is the biggest ever to hit the region (the previous record was a 5.5). After many weeks/months of small tremblors, the area was jolted by the 5.6 mag quake which caused damage to highways, and damage to houses. Luckily there were few, if any, serious injuries to residents.
Geologists have yet to figure out what the cause of the quakes are. These quakes, including the 5.6, are at a depth of ~5km or less, which is geologically quite shallow. Some residents suspect "fracking" and other oil/gas industry activities may have something to do with the tremblors, but this theory would fail to explain why large quakes up to mag 5.5 have happened in the same area in the past.
It could be a combination of the two. The area does lie on what is referred to as the "New Madrid Fault Line" and has produced large quakes in other areas of the country along the fault. It could be that tension in conjunction with fracking or drilling is setting a few things in motion, however I don't want to jump to any conclusions, rather I'd like to see what USGS can come up with now that they have seismographs and other equipment installed in the area.
Most people do not think of the middle-USA as a seismically active region, but it surely is! Earlier this year, a quake in Virginia rattled the East Coast "Like a bell" and sent shockwaves as far as Canada and DC, causing damage to the Washington National Monument that have yet to be fully repaired. Occasional aftershocks do still hit the area from time to time, although those should diminish over time.
The Oklahoma quakes have been ongoing for some time, and it is a wonder that the USGS has not really given much interest to the area. You'd think they'd be pouring over data to determine what's going on. As the area is not known for any recent volcanism, you can probably rule out a volcano, and assume this is purely tectonic... but we just don't know until an adequate study is done.
In the meantime, as I have said before, it's always good to have an emergency preparedness kit handy in your house for just such unforseen disasters. It is very good that nobody was hurt on the highway when it collapsed, and even better still that damage to private property was at a minimum. Stay safe!