FastPictureViewer has an option to automatically rotate images based on their EXIF orientation metadata field. This option has apparently been added to support Windows Explorer, which does not handle this correctly.
Unfortunately, FastPictureViewer then also delivers JPEG files to IMatch already orientated, but keeps the EXIF orientation tag unchanged. Since IMatch cannot know that FPV has already rotated the file, it will rotate it again - correctly applying the EXIF orientation info in the file.
If this happens to you, re-install FastPictureViewer and disable the option to rotate JPEG files automatically.