why does the game dragon age inquisition start downloading during gameplay

Open up Origin and look at the settings. You likely have some things set to automatically update. I doubt it is the whole game downloading just as Ominous implied, it is more likely the patch or an Origin update. But you can turn off the automatic updates in settings, then just update when you want. It is a good idea to turn this off for games at least, so you can find out if the updates introduce bugs before patching.

Note this will cause Origin to say you cannot play games until they are updated, but if you set origin to offline mode you can still play the game.