Can't tell for sure without looking at its code.
There are a number of things that could be causing this behavior (in order of likeness):
- Coding issues: broken/bad HTML code can cause issues affecting the behavior of one or more elements (in your case, the popular list).
- Styling issues: bad CSS coding practices can break sites on different browsers, including mobile browsers.
- Broken Javascript scripts: scripts that alter the HTML of the site (such as lightboxes, custom menu dropdowns, etc.) can also break things.
Now that we know what the problem is, you can switch back to your current theme. Let me know when you do so I can take a look at it, ok?