A Mystery to me, can you explain it?

[Andy Stringer]

...how in the world did I get the background to go black...

Maybe it's because the background was black compared to the other post.

I notice you were in manual exposure mode, hence the identical settings. It looks like you were photographing the shady side of the first post, so using the same settings caused it to be under-exposed. It needed one extra stop to make the two posts appear similarly exposed, but the background would still have been muchh darker than the second post.