Miniature lumberjacks?

Over the years a phenomenon that I've witnessed a handful of times but seen twice today. I'll be sitting in the garden or looking out of the window at long grass with seed heads. One of the seed heads starts to wave about wildly, then it drops a few inches, then a few more, and in several jerks disappears. Whole thing takes just a few seconds. It's like something is cutting the stalk at ground level, or below, and then pulling it down. What can it be? Mouse, mole?