Even the perception of what is a nightmare is different for children. Some infant I knew was waking up for some consecutive nights because "there was a turtle in her bed", and that turtle was "using some of her space, and turtles shouldn't be in my bed". And even being able to explain this after few nights, when she was dreaming and... bored about the turtle, she preferred to scream or cry. Specially because dreams (and nightmares) aren't always a complete package, many times you can only find them to be logical after trying to explain it to someone else.
Ok, so imagine that to a child. Reality and dreams are completely mixed, and he/she doesn't even know how to rationalize about it... I would cry, too :)
One thing that might help is investigating and perhaps changing something in his/her environment: does he sleeps in the crib, or in a different room, so that when he wakes up he's lost? Is the room completely dark or there is some light to help him seeing where he is? Is there some strange shadows, caused by the light? Has something changed recently, like a new toy, that is too exciting, that could always appear in his dreams ?