Why do deferred tasks show as red/overdue?

Using the Defer option for a Task Assignment does not change the due date of the task. It allows a user to place a deferral on a task, which moves it from their main Task Assignments table to the Deferred Task Assignments table. If a user has no deferred tasks, the Deferred Task Assignments table is not displayed.

This deferral feature allows users to visually manage their task assignments page. The user is still responsible for completing deferred assignments by the specified Task Due Date.

The deferred task assignments will appear red once the due date has passed, since the task is overdue.

See related article How do I change the due date of a task?