"Can anyone help me find the exact reason for this?"
I can only guess. So here are two guesses:
1. When you copy large files all of memory gets filled up with casche buffers. If you do not have swap enabled you can sometimes crash when memory is full. Check to make sure that you have swap set up correctly.
2. It is possible that you ran out of space on the partition that you were copying the file to, which should not result in a crash, but you never know. Use the df command to check the amount of space on all of your partitions.
Be prepared. Create a LifeBoat CD.