The Washington Post has more information regarding the firings of the attorneys that will testify before Congress next week.
The White House claims they were not working hard enough to carry out the president's policies. This points to a classic dispute between the presidency, Congress and the bureaucracy. Each has its own ideas concerning the implementation of public policy and the turf to make it happen, which leads to conflict.
There are constitutional issues at stake here as well. Who controls the bureaucracy? The president, due to his constitutional role as chief executive, or the legislature since they established the laws that the bureaucracy implements as well as the appropriations that fund that implementation?
Politics underlies this conflict also. Is the White House complaint based on policy or politics. The former is permitted, the latter is not.