I'm more inclined to believe that our senses are especially on the lookout for attacks from behind, so if we catch subtle cues that someone or something is behind us that we weren't previously aware of (subtle sounds, air disturbances, the diversion of other people's eyes to look at it) we're more likely to turn around to see what it is. Some of the time, the person we sensed behind us is looking at us, but we disproportionately remember those times because it feels more weird and memorable.