It depends on what you mean. I take it that you're talking about a single bend/break session, not the difference between fresh and aged copper, that, at the start it's relatively easily bent but later it's harder to do? I've never tried it, so I can't comment. I don't know why this should be, but it could be (and this is only my idea) a result of flow properties. In the case of liquids, some liquids under shear will actually become stiffer and harder to shear (it's called dilatancy). Perhaps the grain boundaries of the copper (formed when the metal solidifies from the melt) move easily at first but then lock in some way under stress. Must look into it for interest.

I like my horse poop! (If you see what I mean). It's become an old friend,as it were. And now that I've discovered that it comes from Salisbury (falls on knees, pointing in the direction of The Holy City), I shall cling to it (if you see what I mean).