A lot of poor designs can be attributed to sticking with an existing design as changing requirements show us the need for a better one. Oftentimes, an initial design is just a stab in the dark. We might not know enough to make an informed decision but we have to get something done, so we do what Agile says and we code up the behavior that we need right now and not worry about future requirements.
For most teams, the problem comes when they start to enhance that behavior and go back into the code to extend it. Now they’re asking the system to do something that it couldn’t do before and, instead of redesigning a feature to accommodate the new behavior, developers might try to hack in the new behavior while minimally impacting the existing design. But this can degrade the quality of the code when done over and over again in a system.