An event happened in a task linked to the current one that met the requirements for automatically changing the task status. Check the previous task in the task chain. You can find it in the Links tab to the right of the task window. The selected task links will be visible in the Gantt diagram on the Links tab.
Cerebro has rules for changing linked tasks statuses. You can find the rule settings in Administrator window > Task Statuses tab > Linked tasks rules tab to the right.
If you don't need task statuses to be automatically changed, you can unlink the tasks or disable the rules.