Hm, sorry, I still have no idea how this can happen. ??
If I open your mentioned site (even using a fresh browser) I’m redirected to https, too. No chance to visit the page using http. Does this redirect also only appear if WP-Piwik is activated?
Did you add the tracking code manually (without using WP-Piwik)? Does not look like the default tracking code.
Is it possible that there is another plugin you can deactivate instead of WP-Piwik to stop the behavior? Maybe an interaction between WP-Piwik and a second plugin causes the issue. If so and I know about the second plugin, I’ll have a chance to debug what’s happening.