[dainbramaged]

If you’re walking your dog near a salmon stream in the fall, be sure they are leashed - for some reason they love to roll in the rotted spawned out carcasses which smell really bad.

[Boogieman]

Dogs like to roll in stuff that disguises their scent. It’s predator behavior. That’s why if you give your dog a bath and make them nice and clean, the first thing they want to do is go outside and roll around in the dirt and grass.

[blam]

"If you’re walking your dog near a salmon stream in the fall, be sure they are leashed - for some reason they love to roll in the rotted spawned out carcasses which smell really bad."

Yup. They'll roll around in most any kind of carion.

Don't know why.

I found this:

Why Do Dogs Roll in Smelly Stuff? | Dogs Rolling in Poop & Dead Animals