I can't say the reason for "dumping" the duck(s), nor can anyone else...unless you stopped and asked. Beyond that...its ALL speculation.
As you say...you saw a guy disposing of a duck carcass in a ditch, I am not contesting that....or the appropriateness of choosing that site. I am just saying everyone is quick to jump to conclusions.
He MIGHT have been a guide with some ducks he didn't want to clean, OR the duck(s) MIGHT have been in the back of the boat where a fuel tank leaked and contaminated them, who knows?
There may have been several "failures" on his part, or not. You only saw a small part of the picture (entire hunt/event) though...and have tried to "fill in the blanks" around that.
At best...the guy picked a poor place and way to dispose of the duck(s) for whatever reason.