Unwritten rules are based in organization values - often unwritten or even unconscious values.
For example, if an organization unconsciously values "action orientation," then a person gathering a lot of data before starting execution on a project could derail. Or if the organization talks about collaboration but actually values internal competition, trying to collaborate will only get you run over.
When joining a new organization, It's important to ask "How do things really get done around here? What are the behaviors that get rewarded (and not)?" Then ask again after you are onboard.
While unwritten rules can certainly derail those new employees you worked so hard to hire, they can also be blindspots for you about how your own organization works. The values and the behaviors that you think are being followed may be just that - thoughts. Check in with your leadership team, with your new employees, and beyond - to learn if your top-level intentions have translated into behaviors.