Well, based on what I know of dog behavior, I can think of two possible scenarios.
Scenario #1:
You said he was smart, obedient, and well-trained.
Dogs that are startled and frightened by something don’t prick up their ears alertly and then head off deliberately in a certain direction; they flatten their ears, tuck their tails, and bolt wildly. So he wasn’t frightened; he simply heard something coming from the direction of home that caught his attention and caused him to start making deliberate, fast tracks for home.
And the most likely thing he would have heard that would have caught his attention that forcibly would have been the person who did his primary training. You, as a kid, would have been lower down on the social totem pole of his “pack”, but either your mom or your dad, or whoever his trainer was, would have ranked at the top. And a summons by that person would have resulted in his dropping everything and making fast, deliberate tracks in that direction.
So I surmise that at that point in your walk, there may have been a noise that always sounded, to his ears, like whatever his trainer (“Master”) used to mean, “Come!”–probably a whistling noise, since you never heard it yourself (you’d have heard someone hollering, and many dogs are trained with whistles. (Even Daisy the Beagle here at Goose Acres, who has never had a minute’s training in her life, will come and see what’s up if someone whistles.)
So off he went, every time, alertly and at a good speed–your commands to “stop” would have naturally been ignored, since he was perfectly aware that Master outranked you, and Master’s commands took precedence–but when he got home, he found no Master, and was embarrassed and confused, so when you arrived home, panting, after him, you found him looking embarrassed and confused.
And he was in the basement because that’s where the door to the house was left open, and he had clearly heard Master summon him, and that would mean “in the house”, because that’s usually where Master was to be found.
Scenario #2 is that he always simply got tired of puttering around with you at that point in the walk, and hightailed it for home and the food dish, and since you ranked lower than him in the pack, he ignored your commands to stop, but when you arrived home after him, he was suddenly embarrassed retroactively at having left the segment of “the pack” that had been out on a walk. “Uh-oh, mebbe I shouldn’ta done that…”
I like Scenario #1 better myself. 