A while back, hereabouts: http://www.pbase.com/jason_daniel_anderson/image/90872009 a number of these phenomena appeared in the pond ice. Naturally my first strategy was to ignore them, which invariably results in the ignored things going away. Which they did.
However, they've come back, and in much greater numbers -- which invariably things that are upset at having been ignored do. The "thrown sandbag" explanation was evidently a red herring -- the sandbags were removed a few weeks ago. So at this juncture I have lots of holes and no theory to put them in.