Well, every year there are a lot of consultants that are pushed out of MBB. They are not necessarily bad, but the “up-or-out” policy is there, and sometimes one is unlucky (had one bad project), needed more time to develop, or is simply the innocent victim of office politics (yes, this is a reality).
These are not bad consultants. You woudn't believe the ammount of consultants that get pushed out of MBB. And even partners get pushed out. This is Peter's Law on steroids. You can be great but if you didn't make the cut on time to be promoted… you are out.
MBB will often try to outplace these consultants to clients, but some of them want to remain in consulting, so they take offers from the competition.
So Tier 2 firms will frequently interview these great people getting out of MBB.
In other instances, the consultants just weren't happy where they were, or wanted a different lifestyle (less travelling) or to focus on a different industry where the Tier2 is actually stronger. Or the Tier2 is paying them more. Or has a better project (in my case, I left MBB to become EY's #1 strategy employee in my country to setup the new strategy practice, pretty cool, right?)
Or they decided they wanted to quit MBB and go to the industry, just to find out they actually prefer consulting and to return they have to apply to Tier2.