It must have just taken a year or so for the dirt to build up to the point that it created a conductive path. Heh. The rubber boot is there for this exact reason - but it's not strictly necessary, it does nothing for the actual connection at the tube. Really old sets did not even have rubber boots.
I once was working on a TV set - an old one, which required removal of the picture tube to get at the chassis easily (the tube was bolted to the chassis/frame, and supported by it. Very different construction than a modern set). I had done some repairs, and reassembled it, reinstalling the tube. I fired it up, and it was working, but the picture was a little small and jumpy, and I kept hearing this buzzing noise. I look over on the other side of the tube, to find that I forgot to reattach the anode wire - it was hanging from the chassis support, about an inch or so from the tube, arcing through midair to the anode connection on the tube with a nice blue arc. It was enough to make the tube light up and work!
-Ian