GC, I think XT's are just having the same broking house. For example, I use NAB Online Trading. If someone else uses that and we buy and sell, it ends up as an XT (I think).
As to why people sell, well, those that have that large an amount of shares might also have a margin loan, and have made losses somewhere and get called in to sell within 24 hours. They sell what they can. Or maybe they just want cash to buy a boat. :)
