dddd

I think that repechage and Salamandrine have somewhat keyed in on the actual problem, which I have known to occur at proper road bike air pressures.

I'll first get out of the way the possibility that the Araya rims may be relatively slippery along their extruded and anodized tire-gripping surfaces.

But what I have concluded without any doubt is that certain tires, usually those having aged/dry/dirty rim-contact surfaces, are mightily prone to slipping along the rim!
And even blowing off the rim in the case of the narrow old Araya double-walled rims.

I have applied rim cement along certain old tire's bead-seating area to mitigate this, but I can't really recommend this. It did help though.

Don't get me started on tubular valve stem creep, I have too many stories...