Barking Platform 2 Track Renewal

Does anybody know why they replaced a large stretch of ballast at Barking Platform 2 over a few weekends starting sometime in July, to then replace the whole section entirely this weekend? Nearly the entire lengths ballast had been replaced from what I remember seeing last week, it seemed a bit of an odd thing as the section of track was looking a bit tired.

These links to the District Line twitter feed  show them replacing the ballast (By what looks like a giant vaccum cleaner)
Big Vacuum
Removing Ballast

This weekend them replacing the whole lot with track panels
All replaced
Track Panels