If a writer's contract contains a decent reversion clause, chances are that their rights will revert to them once their books are out of print or otherwise unavailable for six months or more.
This was fine when offset printing was the only reasonable method of printing available: but the advent of POD printing means that books can continue to be made available even when the publisher is no longer in possession of any physical stock.
Consequently, what used to be standard reversion clauses can now be impossible to invoke, and contracts which don't differentiate between printing technologies can allow publishers to hang on to books they are making no attempt to promote.
In the United States, Simon & Schuster tried to take advantage of this potential difficulty with little success. Now Random House in the United Kingdom is flexing its muscles in the same way.
While relatively few books enjoy a second sale after their rights have been returned, I find it disconcerting to see this potential source of income denied to writers.
If you want to read more on the subject this is an excellent place to start, and Googling "Simon & Schuster" and "rights grab" should find you everything else you need to know.