If you have the JC Higgins Model 50 like I do, I can tell you that prior to my inheiriting it my dad had the same infrequent complaint. I have NEVER had the problem with the same gun. I think its because the wing safety, when the gun is carried on the right shoulder, catches things thus unlocking the bolt. Now why the bolt opens, I am not sure. Might have a smith look over the wing safety, maybe its worn and needs replacing (piece of cake).
I don't think you have a problem with the gun. You need to carry it differently. I HIGHLY recommend a Safari Sling, with which you can carry the gun in front of you and keep an eye on things.
Congrats on having a very fine rifle!
RR