I'd have thought if it's the front of your shoulders there'll be a grip and pulling element affecting you. Like Ali says the position of your arms and the bike in the vid show you're pulling to keep the front end up rather than balancing, you should be able to hold the balance on the rear wheel for a second or two without hopping if you're in the right position.
Another thing that I think will be adding to it is it sounds like the rear brake is slipping slightly which would cause you to squeeze the bars and brake lever harder than you need which again will be adding pressure on to your bicep and shoulders.
I can send you a video showing a really easy stretch for the front of your shoulder if you want? Should help a bit, let me know