Website URLs seem to refresh in the background
Many browsers do prerendering for often-visited sites.  It's possible Safari is preloading the URL as a top hit because you visit that URL a lot. Some browsers also prefetch sites to create image previews. 

HTTP GETs are supposed to be safe and idempotent according to the W3C standards.  Many browsers (and proxy servers) make the assumption that it's safe to trigger and/or cache GETs. POSTs are supposed to be used for effect-inducing operations.