The lip curl (Flehmen response) is a deer behavior especially common during the rut, exposing the vomeronasal organ (Jacobson’s organ) to scents, especially those of a doe in heat.
While this behavior is not unusual, it is different from the many images captured of the same old buck simply standing and looking.
The bokeh buck is a want-to-be contender.
He doesn't stand a chance against this clearly superior buck.
As I mentioned in the other photo of this buck, a few minutes with the right subject in the right light and location scenario can result in a lot of nice images on the memory card.