The attrition rates are high due to newly qualified’s choosing to leave and not due to them being forced out.
Newly qualified’s in the Big 4 start getting head hunted even before they are fully qualified. After you do the final exam, literally as you are walking out of the exam centre, recruitment agents are waiting outside and handing you their business cards and then the barrage of emails and calls offering you jobs come along. The partners are aware of this and quite accepting of the fact that once you are qualified, you are very marketable. The reason people choose to leave is more than likely due to the salary difference upon qualification. The banks and some positions in the industry can offer salaries that are about £20K more than what you would earn if you stayed in the Big 4 after qualifying and a lot of people are attracted by this and choose to leave.
Just to butress my point, have a look at the links below:
Newly qualified (within Accountancy)
Newly qualifed (in a bank)
My point is, the Big 4 invest quite a bit to train you and would do their best to retain you though this isn’t always possible as you are only tied in for the initial 3 years training contract.