Middle tier clustering is just a cluster that is used for service the middle tier in a application. This is popular since many clients may be using middle tier and a lot of heavy load may also be served by middle tier that requires it be to highly available.
Failure of middle tier can cause multiple clients and systems to fail, therefore its one of the approaches to do clustering at the middle tier of a application.
In java world, it is really common to have EJB server clusters that are used by many clients. In general any application that has a business logic that can be shared across multiple client can use a middle tier cluster for high availability.