The rise in game-based movies is not a fad, but it is the natural evolution of things. Before games, movies were largely based on books. As games improved in visual fidelity, it became compelling for Hollywood to produce stories around them.
The game companies themselves seemed to start releasing movies in the 1990s (Street Fighter II: The Animated Movie comes to mind). As those companies starting generating more revenue from their game sales, it became possible to fund their own movies.
The Marvel universe has a lot of characters and stories. I like how they are trying to manage and interweave them in an organised fashion although I'm sure it'll get old at some point.
Having grown up with the Marvel cartoons from the 1990s, I can affirm this is something they've always done. DC isn't much different.