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 task "Properties", "Links" tab. Check the task marked with "From". 

Cerebro has Rules for changing statuses of linked tasks. You can find the rule settings in the "Administrator" window in the "Task Statuses" tab, in the "Linked tasks rules" tab to the right.

If you don't need task statuses changing automatically, you can unlink the tasks or disable the rules.