Hmmm.. good solid 531 shouldn't break easily, including street riding. Is there a chance that it's not 531? If it's an old high-end frame, it COULD be 753, though that's pretty rare and not quite period-correct. 753 is pretty delicate, though.

I would say perhaps the tube has rusted out, but I don't believe lugged construction needs holes at tube junctions the way welded does, so there shouldn't be any way for water to get inside the tube in the first place. It shoudl be 100% virgin 70s air.

But I could be wrong.

I see that the crack seems to line up very nicely with the furthest inward extent of the lug shaping and I wonder if there is just too much flexing in the frame which gets expressed right at the point before the lug adds sufficient rigidity to dampen any up-stream flex. I believe this could be true if the tube wasn't properly mitered when fitted to the head tube.

Ultimately, this is all non-expert speculation though. Given that it's happened while you've owned the bike and is presumably the result of your riding, I would consult a frame builder.

It's also true that everything eventually wears out. This could be your frame's time to go.