Why? My understanding is that EasyJet as the biggest airline at Gatwick wanted (understandably) to have all their flights from one terminal and told Gatwick they wanted the North Terminal - taking Virgin with them - so BA got kicked out into the South. I'm quite happy as I prefer the South and tend to use BA rather than EasyJet.
Personally thought it was BA who wanted the south.
They are paying for all of Virgins relocation costs - lounges being rebuilt/compensation for loss of revenue for the V-Room being shut and comp for the passengers.
I guess that maybe to speed things up, but it's a bit odd!