Tracking Code in wp_footer

Piwik is using an asynchronous tracking code. So browsers will not wait until the piwik.js fail is loaded before your site is shown and it is not recommended to move it to the site’s footer. If you like to do so, you have to switch to a WP-Piwik version earlier than 0.9.9.0. (With 0.9.9.0 async tracking was included to stay compatible to Piwik 1.11.)

See http://piwik.org/docs/javascript-tracking/#toc-where-can-i-find-the-piwik-tracking-code