It's crazy how someone could walk into your office or closet or some kind of private corner of yours, pick and make use of your personal property without informing you. This irresponsible habit has found it's way into cryptos.
A programme called coin hive enables websites to mine remotely with their visitors machine.
Some peer-to-peer sites (piratebay, extratorrent ...) have implemented this script which allows them mine Monero with your computer anonymously. 
The question now is; could this act be justified?
there now is a chrome extension to block background mining https://chrome.google.com/webstore/detail/no-coin/gojamcfopckidlocpkbelmpjcgmbgjcl
Why is it anonymous? Half of articles here are about this.
you only just got to know about it. It always had been done without your knowledge.