> "For more senior engineers, it can happen because they like to work on their own and may be overconfident in finding solutions. It can also happen if the team culture is toxic and engineers fear getting criticism early in the design process."
Not saying the above statement is incorrect but here is an alternative explanation that is also viable: Senior Engineers are often hired into large large projects so that the company has the capability to address emergencies or modify the existing system accurately as it is often more difficult to work on a large complex code base than to build a new one from scratch. Those engineers sometimes never get to work on greenfield projects and "doing too much work on their own before looping in others" is a way to scratch this itch without the opportunity being taken away prematurely. It also offers chance to produce memorable work as nobody remembers the set of 3 point tasks you completed ten sprints ago.