Ok, check my screencast https://v.usetapes.com/p7eikpeXBI
Easy to understand why it does that. You have elements that have interactions making them slide from outside of the site to inside. So when the page loads, they are outside the window right? But as your sections aren't set to overflow:hidden (see behind), the site width is taking them into account. If you scroll the site entirely, the bug disappear because everything is in the window, all the interactions have played. So select the sections you have sliding elements in and set them to overflow:hidden, it will hide the content that is outside the section, thus not taking these outside elements to calculate the site's width.