When wounded why will certian ducks submerge into the lake, grasp onto a reed and drown its self?

i am in the mississippi fly away and it is usually the green winged teal doing this. You will bow it out of the sky with some #2 steel and it will hit the ice cold water with a forcefull splash. It will leave a blood trail in the water so you can follow it into thick reeds and here is where you find it around 2 feet beneith the surface holding on to a reed while drowing it self. Sometimes it will release its death hold once it has expired, other times we have to take a oar and givee it a good smack on its heat for it to release.