avast detects JS:Decode-AKQ [Trj] here, but how the site was being hacked?

See example: http://killmalware.com/lasreglasdejuego.com/
Read about it here: http://jeffreysambells.com/2012/12/12/anatomy-of-a-hack
and http://stackoverflow.com/questions/16929895/is-this-site-hacked
We see Jeffrey Sambells attack descriptions goes back to 2012.
See: https://www.virustotal.com/nl/url/42b929224ee595e65f8ad3c4fef09e35081429103464ca9a3c75c7f8aa25b617/analysis/
and http://sitecheck.sucuri.net/results/www.lasreglasdejuego.com
Outdated CMS: Web application version:
WordPress version: WordPress 3.1.3
Wordpress version from source: 3.1.3
WordPress theme: htxp://www.lasreglasdejuego.com/wp-content/themes/mystique/
WordPress version outdated: Upgrade required.
Outdated WordPress Found: WordPress Under 4.0

pol