Please check the Audit Trail report to see if this change is being logged.
You can set the Playback Level to High and enter the search string: Update CommCell properties

You should see a record like this:

Then you know which user is being used to perform the activity.
Question still is, how is this performed.. Thinking about how to identify this

Can you also check if a DR backup is performed around the time the activity is disabled?
If so you might have a Database corruption, when the Disaster Recovery Backup detects this the CommServe job activities are disabled automatically.

Dear @Jos Meijer thanks for your answer, yes you are correct i see a problem with SQL as follow image:

so how can i solve this database issue

Good dat the reason has been identified 
To be honest I would contact support, they will get you back on track asap.
Analyzing the issue through this forum thread will unnecessarily delay your solution.

@Jos Meijer thanks alot

Hello All

nearly,I had the same problem,how to solve this problem?

Thanks!

@sc.zhao 

In CommserveDR.log, if you see consistency errors in the DBs, please raise a case with support for further analysis and provide the right POA to solve this problem.

Example: