I'm guessing you have an iPad 1 and this could be a ram/memory issue. Make sure all your apps are closed completely. Do you have a lot of location services running? Safari and whatever websites you view can take up a lot of ram. Go into Settings - Safari and you can delete history and cookies. Then have a look at the advanced tab here also, then Website data. Safari collects all of this data from every site you have visited, delete this, as you will see it can add up. Try your download again and let us know how you go.