I have heard from a few different sources that it was the new pilots that were sent home, allegedly both were struggling to fly to the standard required, and there was some discipline issues involved as well.
In fairness having watched countless programmes about the Reds training, many previous pilots said flying with the team was very different to their operational squadron flying, and it was almost like learning from scratch.
The Hawk T1 is an older type, and pretty basic compared to the modern frontline aircraft, and some find the transition very difficult or worse.
The Reds have to get their PDA for the season by a certain deadline, and if elements of the team are still not ready, then at some point a judgement has to be made.
The alleged discipline issues are probably where the media decided they would just run with something, even if it wasn`t true.
This is one of those situations were the RAF may have been better not to have said anything, other than it was for operational reasons that the team would display as a 7-ship this season.