I was told a long time ago it was from the natural desire to move their feet while grazing that some horses will paw or move a leg while eating grain. A lot of young horses will grow out of it. My young horse at the time did it but grew out of it, and I’ve got a new 2 year old now that is doing it as well. He rarely paws otherwise.
I’ve heard this as well. Also something about pawing to find food is a natural behavior, such as pawing in the snow to uncover grass. And the action and thought process stimulate secretion of saliva.
My guy did it at first (when I got him at 4). It was like left over foal behavior. He would also get a bit defensive if you tried doing anything around him at feeding time. Trying to block you with his butt and so on, while pawing. Again, looking very foal-like trying to act all big and tough so he can stake claim on the food. So, I started working on him being ok with me in the stall while he was eating. I could pick poop, dump water buckets, eventually do blanketing/unblanketing, and some light grooming/picking feet/wrapping.
Once he got over being defensive about his food, the pawing stopped. I saw it as more of an insecurity. Sure, he could dig through snow for little grass shoots, but that type of pawing was a different behavior.
I have one that does this too; a 4yo mare, born and raised on the farm and has always had adequate food. But she’s a diva, and will paw her feed-tub until it spills grain, and then eats the grain off the mat. She’s messy, but always cleans up after herself, and it’s not annoying. She doesn’t paw waiting for the grain, only once she receives it. She’s also one that will limbo under or between stall chains, stands on the gate waiting to come in, and loves to spread her own fresh shavings out after her stall is cleaned.
I have another older mare who’s obnoxious about pawing until she gets her grain. She hates that she’s #3 to get it. That habit bothers me more, honestly. It’s loud and obnoxious.
Not pawing, but I have a young gelding who likes to chew on the edges of his feed bin while he’s eating his grain. He’s done it his whole life. I’m not sure where the habit came from, but it is funny how some horses have quirks about eating.