Is it possible they tipped a bowl over? Are you sure it's food pellets and not poop pellets? LOL! I've never had guinea pigs hoard food in this way (though many have tried to drag the food bowl along with them by biting it and backing up to try to keep it all to themselves.) Because they don't don't have dexterity in their paws like mice/rats/hamsters, it's a lot harder for them to carry food and deposit it somewhere else. It's also less wired into them than, say, a mouse or hamster, because wild mice and hamsters have a territory with burrows where they can hide food, whereas wild guinea pigs basically walk and graze all the time. I'm interested if you can catch them in action to see how this is happening! Let us know if you figure it out!