Do the short to ground more than once, or hit it and leave it connected for a few minutes, as the tube still has some charge within - like a lot of HV devices, it can be shorted to ground for a second or so, then with the migration of electrons from out of the insulation and other structure, will build a (smaller) charge to discharge through the next available path...
I, too used to throw rocks/shoot tubes...the implosions could be pretty amaziing. The shot or rock propogates cracks in the glass, the structure fails faster than the glass (now shards) can accelerate and the vacuum causes the glass to all head to the middle, and then to pass through the incoming opposite side's also incoming glass, thoroughly shrapnelling the stuff. Do it inside a room, and the floor and ceiling, and the walls will have sharp shards embedded, if you started with a big tube, bare...