Following are the answers to your questions:
1. Rate: Both maximum rate and enforced rate were changed to 5 QPS. I was only using default queue and set 20 QPS on it.
2. Task: All my tasks run on frontends
3. Admin Log: Yes, I am the only developer uploading changes and there is no activity in admin log when the setting was changed.
In addition, when I tried to change the setting to 100 QPS this morning in order to drain the queue, it was always changed back to 5 QPS about 5 minutes later after the deployment. Finally I found a workaround to create another queue with the same setting which stopped the madness of automatically configuration change.
Actually task queue has been causing service interruption to me in the past 2 days. Yesterday, the default queue suddenly stopped accepting any task with name, which I had to workaround in order to bring my service back to normal state. I was too tired to write a post here about the issue after I finished live site support yesterday. Since you are you investigating, would you please also look into this issue as well? I really need the feature to add task with name so that I can avoid duplicated task processing.