Why does dog pull out tufts of grass by roots?

Both of my Golden Retrievers (but not our other dog) will catch their tennis ball, then drop it in the grass and begin frantically to bite into the turf .  They'll pull out two or three mouthfuls of grass, roots and all, and then snuffle around for a moment in the dirt they have exposed, and then pick up their ball and walk away.  At first I thought they were trying to bury the ball, but they don't even make a move to put the ball into the hole, much less cover the hole back up with their nose or anything.  Then I thought they might be eating grubs in the soil?  Has anyone ever seen this behavior, and did you ever get an answer about what causes it?  The dogs get vegetables in their diet.

[ginger899]

I know a dog who does exactly this. A big black farm dog who visits us and has become our friend. I always thought it was a "let's really kill that ball!!" thing. (they have caught it, now they are killing it)  My Misty doesn't do this, but he does.

It doesn't seem to do him any harm, in fact I always think it must floss his teeth really well. The only bad thing is occasionally he gets strands of grass stuck between his front teeth. Then it looks really dramatic because he gapes wide, paws at his mouth. It really scared me when he first did that. But when I pulled out the strands of grass from his teeth he was ok