Is it a nutrition deficiency?
Usually it’s because they’re constipated or having trouble coughing up a furball.
Fibre (eg Weetbix and warm water, or cooked green veges with dinner) will usually help.
Make sure your dog doesn’t eat any grass or plants that are poisonous or have been sprayed with chemicals (fertiliser or weedkiller).
My dog ate my grass one night, and couldn’t stop smiling.
I’ve heard it is their way of inducing vomiting if they have an upset tummy. Very natural thing for them to do. If they do it a lot however you might want to look at their diet more closely to figure why they are constantly having an upset tummy.
They do it so they can come inside right away and make sure the new rug is sufficiently well coated as to have an uniradicable smell.
My wife’s dog ate lettuce every day. Before his kibble too-- if you put the lettuce underneath, he’d pull it out and eat it first.
Pretty damn civilized, if I do say so myself.