To understand the Undone work, one must first understand the Definition of DONE. The definition of DONE as pre-Scrum Guide states, "When a Product Backlog item or an Increment is described as DONE everyone must understand what DONE means". In other words, the DONE means RELEASABLE. UNDONE means the pending work on the Increment that stops it from being releasable. Pending work in this context refers to work pending as per the Definition of DONE.
The probable cause of the UNDONE work is not adhering to the Definition of DONE, which is subjective. When a new Scrum Team gets formed, the team defines their Definition of DONE with their best possible knowledge of the Product and Development team's skills. In this scenario, it is not sure that the Product Increment the Development team creates is releasable. It could give a different definition to the Undone work now.
Consider an example: A scrum team is working on a Product that falls in the regulated environment. During the Product's initial stage, consider the Product Owner does not know the documentation needed to get the go-ahead from the regulators to release the Product to the market. Though there is no pending work on the Product Increment as per the initial Definition of DONE, it is still not feasible to release it.
Here, Sprint's UNDONE work is the work required to be completed between Increment and make it releasable. As the team matures and learns more about the Product, the Definition of DONE evolves, which could ensure the Product Increment is releasable. UNDONE work could delay the Product to be released to the users and hence delay the feedback cycle.