This is the explanation:-
"Running all day on Easter Sunday (31st March), the unusual London Overground service between London Bridge and West Croydon is needed because of Network Rail engineering works, which would have meant no trains to or from London Bridge operating through the Sydenham corridor on Easter Sunday.

Although TfL can run a shuttle between New Cross and West Croydon, as people will want to get into central London, TfL decided to take up an option to run trains into London Bridge for the day so that people could get into town and home again.

So, for one day, it will be possible to catch a London Overground train from a station they would never usually be seen at.

Which I am sure will utterly thrill anyone with even the slightest bit of train geekery in them.

For the record, TfL has confirmed to me that this has nothing to do with a suggested permanent service to London Bridge, and the Easter Sunday special is only happening because of the engineering works."