It wasn't the brine.
Was the bird already brined/enhanced (many are).
You have a 1/2 strength brine at best. A "normal" ratio is about 1 gal to one cup of table salt. Your ratio is 1 gal to 1/2 cup. Kosher salt by cup will be less salty then a cup of table salt.
FYI, I brine mine for 48 hours with no one commenting.
How about the rub, is that salty to you? How much, maybe you over rubbed it.
Check out my link below for more Brining info