It’s a function of page width, which in turn is a function of the images in the thread.
If an image is less than 1000 pixels wide, ZBC’s text will wrap like normal because the image width is narrower than the default page width. But any image wider than that will stretch the page – affecting ALL text on that page. (Even that in posts other than the one with the wide image.)
At one point ZBC used to show wide images as pop-ups so as to avoid this issue. However, there were too many complaints from users and we reverted the current system. So if someone posts a wide image, you have to hope that they (and everyone else on that page) insert carriage returns manually in order to prevent the need to side scroll. Unfortunately, almost nobody does that.