Why does the handler fire several times after page resize?

Question

JavaScript

Hi all! Actually there is a menu that is vertical on mobile screen sizes, and horizontal on desktop. When resizing the window, I add or remove a handler that collapses / expands the submenu on mobile screen sizes. But after resizing, this handler fires several times. Explain, please, why so? The code is here https://codepen.io/EptaScripter/pen/RwpdWeo