Would you have scratched the tube, it wouldn't have taken 20minutes to go flat (95% of the time you simply cannot inflate a tube that is scratched during installation).
This is mostly some sort of storing issue, you usually got that when hanging your inner tubes on a hook. It's a rather common thing to observe in second-hand tubes that don't go directly from the box to the wheel, the rubber get compressed and once inflated you get this weird look.
Or you can, but it's rarer nowadays, have this in a brand new tube, just being caused by non homogeneous rubber distribution, it happened to me sometime ago.
Most of the time and from my experience if the deformation is not to pronounced (and it does not seem that way here) it should not cause any problem since the inflation will simply fit the shape of the tube into the tire.
It's uncommon but with bad luck the deformation could cause a hernia to happen but then again with a hernia your tire would have not taken 20 minutes to go flat (usually goes boom and flats out in 2 seconds).
I would go with deffective tube rather than wrong installation here.