It looks like the Event Cart is not positioning itself correctly on the page. It seems like something on the event page is making the page elements wider than the screen, which could be causing the issue because the screen is too wide for the mobile browser.
Upon further investigation, it seems as though your site has two header elements, which may be affecting things for mobile browsers. Here’s a screenshot of what I see: https://take.ms/GyvVS
It looks like the first header element might be empty but it could be affecting the page sizes when the screen is displayed in a mobile browser. Also, it looks like you are using Beaver Builder, so you might be able to remove the extra header element in the page builder.