A recall that works reliably fails completely the moment a dog finds something dead. The behaviour that follows is deliberate, targeted and remarkably consistent in form.
The movement is precise, not accidental
Dogs do not fall into these substances. They approach, lower the shoulder and side of the neck first, and work the area behind the ears and along the cheek into the material.
The head and neck are targeted far more than the flanks or belly. That selectivity is the strongest evidence the behaviour has a function rather than being incidental play.
The same movement appears in wolves and other wild canids, which rules out any explanation depending on domestic life.
Scent disguise as an explanation
One long-standing account treats the behaviour as camouflage. A predator carrying the smell of carrion or herbivore dung is less detectable to the animals it hunts.
The problem is that dogs also roll in things no prey animal would recognise, including perfume, engine oil and household cleaning products.
Disguise may still be the origin of the behaviour without explaining every modern instance. The trigger appears to be novelty and intensity rather than any specific class of smell.
Carrying information back
The competing account runs the other way. A group-living animal that finds a carcass gains from bringing news of it home on its coat.
Wolves returning to a group after rolling are investigated closely by the others, and the group sometimes travels back along the returning animal's route. The coat functions as a message.
This fits the targeting of the neck and shoulders, which is exactly the region another animal investigates first during a greeting.
Why punishment does not reduce it
The behaviour is self-reinforcing. The reward is delivered by the act itself, within the same second, and no consequence arriving afterwards competes with that.
Scolding a dog on its return teaches only that returning produces trouble, which damages recall without touching the rolling.
The workable approach is management: reading the ground ahead, recalling before the dog commits, and building a reliable interrupt cue away from any temptation before testing it near one.
The washing question
Frequent bathing to remove these smells strips the coat's natural oils and can leave the skin dry and irritated, particularly in double-coated dogs.
Spot-cleaning the affected area rather than bathing the whole dog is usually enough, since the material is concentrated on the neck and shoulder.
Rolling in anything with an unknown chemical origin is a different matter. Contact with unidentified substances, particularly oily or solvent-based ones, is worth a call to a veterinarian before scrubbing, because some are absorbed through skin or ingested during grooming.