why does Javascript only work on direct page load

Question

I have learned a lot through SO because, normally, if a question is asked, the root cause of the problem becomes very evident. In the case of this SO question, the proposed solution doesn't make it clear (at least to me) "why" a page will load correctly if a direct url is used but won't load correctly if it's referenced via a hyperlink. I am running into this same issue and instead of just adding a fix/solution I'd like to know why it's causing a problem in the first place. If a url is placed in a browser tab how is that different from the exact url being hyperlinked? Could someone elaborate to help me understand what is occuring? Thank you.