The north (near) side of the plug was put in tension by the wind. The crack started there and progressed rapidly over the cross section until the fracture was complete. Do you see the small cracked piece near the south (far) edge? That’s what’s called a compression chip and is commonly found on the compression side of brittle materials that fail in bending. Even if the post had been taken away, leaving only the piece in the ground, the compression chip would have told me which way the pole fell.
There are some other interesting things we can see in this failure. First, the concrete looks to be pretty strong. The clue to this is in the broken aggregate. Weak concrete tends to break around the aggregate; strong concrete tends to break through it. Of course, if the aggregate itself is weak—and there’s always some weak aggregate in concrete—this can be a false clue. In this case, though, aggregate of various colors and types are broken, and that’s a pretty good indication that the stronger aggregates as well as the weaker ones have failed.
Alternatively, Dr. Drang, you could have just painted the post.