I think a potted history of Scottish Citylink would probably answer a lot of the confusion on this thread.
By 1997 it had ended up in the hands of National Express (100% ownership), which was fine until NX won the Scotrail franchise - at which point the Monopolies & Mergers Commission ordered it to be sold off (NX having a monopoly on public transport between most of the major cities). So, Citylink was sold to ComfortDelGro (again, 100% ownership).
Stagecoach then gained a 35% stake in 2005, as part of the deal to expand Megabus in Scotland and work with Citylink rather than directly competing - this included giving Citylink access to some cross-border Megabus services. Indeed, operationally there is a sub-section of Megabus (Megabus JV) that's involved with all the joint routes. Incidentally, the MMC also got involved in this tie-up, which is why some routes were sold to Parks of Hamilton.
So, the sale of a minority stake in a Joint Venture back to its original owner, along with a wider network of interconnecting and similar style routes is hardly surprising - to me at least.