They're only down there for a very short time and they don't have a lot of nitrogen stored in their lungs, so decompression sickness is usually not an issue. I think there have been a few cases, especially with repeated, very deep dives. But nothing you have to worry about as a normal hobby freediver.
Passing out at the surface is quite common though, but not due to decompression sickness. It's the lack of oxygen that can happen when you have learned to completely ignore your urge to breathe and then stay down for too long. The reason why you pass out at the surface and almost never at the bottom, is because the water pressure compresses the air in your lungs. At a depth of 10 meters (30 feet), four liters of air in your lungs are compressed down to two liters. This is basically "concentrated air", which contains "concentrated oxygen". If the air is compressed to half the volume, it's like having twice as much oxygen in it. Then, as you ascend to the surface, the air in your lungs expands again, turning the concentrated oxygen into regular oxygen and then it's just not enough anymore and you pass out.
That's why it's recommended to always have someone with you who stays at at the surface and who can step in if you pass out, keeping your head above the water. It has never happened to me, but I'm really a beginner and I still have an urge to breathe that forces me to go back to the surface after a short while.
