In Alonso's case, he said he wanted to go back to Spain - that was the only reason RB started to sniff around gareth barry.
In Agger's case, he has averaged less than 20 games per season since he arrived. Any good manager would jump at the chance of selling a player like that for good money. BR won't be able to build a solid, consistent defence with JC on the decline, SC still unproven and DA in and out of the team all the time.
Unfortunately, BR has been severely restricted of funds thanks to his predecessor spunking 3 years worth of transfers on some very average players. BR has no choice but to sell if he wants to change the squad so getting rid of an injury prone player for such a huge fee is a no brainer.