What is the normal descent rate for airliners?
There are a few possibilities, there's no way to know which is correct without more information:


There could have been an emergency on board. If someone is showing signs of a serious illness pilots will get the plane on the ground in a hurry to get the person medical attention as soon as they can. Or there could have been an issue with the airplane or a passenger, like someone very drunk and unruly in first class, say
As @falstro said it could have been a slam-dunk approach, where a controller directs airplanes to do steeper descents to solve sequencing issues. Pilots know if they say "unable" that they probably won't get another approach for awhile, so they'll accept a steep descent to get on the ground sooner
It could have been a request of the pilots for a non-emergency reason, like them trying to make up scheduling time, or to stay within fuel limits