Hi there,
This sounds like a question that is best suited for the SDK category, so I moved it there.
The reason for the problem is this:
There was a bug early on where windsocks faced 180° opposite of what they were supposed to. To compensate, a lot of third-party airport developers modified their windsocks to be 180° opposite in order to compensate for the bug.
Unfortunately, when this issue was fixed, it was then incumbent upon every developer who did the workaround to revert their windsocks. But not everybody has.
As for how to edit the BGL files, I will leave that to someone who actually knows the SDK (not me). But if you have third-party airports with this issue, I would strongly suggest contacting the developers and telling them to reverse their windsocks.