In the jobs I've worked, determining the most important tasks was done at the start of sprints in collaboration with management. Sometimes priorities would chan

In the jobs I've worked, determining the most important tasks was done at the start of sprints in collaboration with management. Sometimes priorities would change mid sprint. (Which shouldn't happen but that's another discussion.) Starting a task that hadn't been explicitly prioritized would get anyone on the team in trouble. It's never been difficult to get new work approved (just ask) but it had to be approved.