To prevent an electrical malfunction (albeit rare) from becoming an electrical fire. Any piece of electronic equipment is a hazard to be the source of an electrical fire. Securing current removes some of the problem. That is why. (Yes, malfunctions that severe are very rare).
There is no checklist that I know of that would direct a crew to put a transponder into 'STBY', which incidentally, does not remove power from the unit.
Any electrical smoke/fire checklist has the crew de-power/isolate busses via the elec panel and/or circuit breakers, not individual components (unless it is definitively known as being the offending kit).
It's obvious we have a lot of armchair wannabes on this thread. Sit tight, the relevant (and knowledgeable) authorities are working overtime on this. They know leagues more than any one person on this site.