I've mainly seen this happen when the site/application is very product/design (non-tech) driven and the developers pull themselves and the code into contortions to deliver very exacting requirements. The goal is to deliver a pixel-perfect result at the end of the sprint, and the unfortunate waste product is the mess of CSS, markup and JS required to deliver it: which then becomes very fragile technical debt that can only be tweaked rather than refactored lest the whole stack of cards collapses. It makes little difference whether the team started out with a UI framework or not at this point.