well people you answer too fast! thank you!
maybe hold one handed sword with both hands is more a necesity,
put atention in this concept art: http://2.bp.blogspot.com/_4lrm97dBqNo/TUFlw5fTXtI/AAAAAAAAAY8/kWbMCXwYAKc/s1600/artofskyrim_010_argoniandragon.jpg
as you can see, the warrior drop away his sword (accidentally), well maybe you can say "my ass thats just concept art!" well you′re right but is a good theory, maybe now you can release the shield (due the attack of a dragon or some other kind of inconvenient) and instead of collect it back, in the middle of the fight is more prudent just hold your sword with both hands and defend yourself.
supporting this theory is also the spanish magazine "when a dragon lands the ground rumbles and shakes"