The primary meaning of "straddle" specifically has to do with the positioning of a person's legs in relation to something, as in the following pictures.
Straddling a stream (or a tiny river, as explained in the source):
Straddling a fence (a common metaphor for indecisiveness):
The best way to understand "straddle" is not with a definition that tries to encompass all the many ways the word can be used, but with its primary meaning. People commonly extend the primary meaning metaphorically in an endless variety of ways to create new and different meanings.
It often helps to consider the etymology of a word to understand its present meaning. Historically, "straddle" is related to the word "stride". This connection shows itself in the strong association between "straddle" and legs, even though people often use "straddle" to mean things that have nothing to do with literal legs. See also "straddle stretch",
"straddle cuddle", and especially "straddle option".
So, even though I have no idea what a compost windrow turner is, my experience as a native speaker leads me to guess from the paragraph that you quoted that it probably has something analogous to a person's legs, and that these "legs" stand at various places around the perimeter of a windrow, with the "body" of the turner above the center of the windrow. (I don't even know what a windrow is.)
The pictures are from: "Straddle the Columbia" by the Bonneville Power Association, and Gravity's Rainbow.