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Thanks all for the feedback. Naturally, I was looking to exonerate myself but I guess there was a fair chance that some how during my hike managed to hit the release button and the lens bolted. I've used this lens countless times in between the two incidents and never had a problem. Maybe next time I'm hiking I'll place a piece of tape over the release.
I think that is the wrong approach. It would be like putting a drumskin over it or tensioning it so that a touch would push the button down more than usual and might release it.
I think you need to make sure that you don't have any extra straps or cords or bungees or strings or iPod wires or anything that might hit it.
If I have a camera over my shoulder, I have my hand hovering around it or on it or on the strap to protect the camera and to be aware of any funny shocks, etc.
I never strap my camera to my chest.
When walking or hiking with a tele-zoom, I carry the combo by the lens. You need to be aware of everything around the camera and be sure that nothing hits it. If anything does, stop, check it out, make sure the lens / camera is securely mounted and move on. Be especially conscious of stray fingers. Do you use the Depth of Field preview button? That can sometimes be mistaken for the mount release. Awareness is key.