TLDR: Because paparazzi
Taylor Swift is probably the most famous pop star in the world - which means she is a target for the paparazzi.
In a Tumblr post Swift explained to fans that photographers, rather than heatstroke, were behind her recent decision to go go hiking backwards, writing:
I saw the guy with the camera and wasn’t in the mood so I hiked the whole trail backwards and my security told me when to make turns.
It's not the first time Swift has turned her back on the paps. Earlier this year she was filmed walking backwards to her car to avoid photographers. That video, coupled with Swift's full Tumblr post, are actually quite sad.