I watched a walnut tree lose all its leaves in about ten minutes one time some years ago and I was wondering if this is something that is normal and if not would there be a simple explanation for it.
How could this happen?
Depends what time of year it was - if it was fall (autumn), the air had been still and cold, the leaves had already turned, and a wind or strong breeze arrived, it's entirely possible all the leaves would fall off in five minutes, never mind ten.
Even if the tree had been sick or very short of water, and the leaves had turned to brown, a sudden wind would dislodge them in short order. If you're saying the leaves were green and healthy and growing, and the tree was healthy, then that's a mystery... so yes, more info, in that case.