This glitch can also happen if you just interact with certain people/signs/objects before walking back to the route. The trainer battle that can occur this way usually causes the effects that you had.
The P.A. system going off and the text telling you to go back triggers the Lass battle. If you had talked to the guy to the right of the Lavender Volunteer House before entering Route 8, you would have encountered the Gambler on that route that's used for the Escape Glitch.
If you had lost to him the first time and returned to a Pokemon Center/MOM's house/Pokemon League, you would have encountered a second Gambler as soon as you appear, (same Gambler from Route 8) but your start menu would still not work. Going back to the route causes the same effects to occur. Messing with scripts and people before entering a route causes things other than the start menu to appear. This glitch causes a script to trigger and close before the encounter starts (or in the case of triggering a trainer, doesn't.)
This is the same reason why you can get the Master Ball early from Sliph Co. It just happened to trigger the event. I've experimented with this glitch and its effects before using Pokemon Yellow in Viridian Forest. I once made an invalid script occur causing a mass of glitch text and sounds to appear (even some glitch text going half way up the screen.) It took a while, but it finally ended with a girl's text from Pokemon Tower properly terminating the script.
What's also interesting is that if you reset or shut off the game before entering the route if you saved after fighting a trainer/Pokemon is that nothing will happen after the script was done triggering. (Start menu, etc.)
The reason why a new glitch did not occur when you took the last step from the Safari Zone into Route 8 is because the Safari Zone recall script triggered before the Escape Glitch's so to the glitch, you, haven't arrived at that route yet. Don't get discouraged though, you may still find something out there, although you're going to need to get creative as R/B/Y is getting pretty dried up in terms of new glitches now.
The reason for the trainers appearing I cannot say. Possibly it calls the last script identifier to have been done on any map and activates it when you enter the area. Hex 00 must call for the Start Menu as this is normally seen if nothing has been done. If a script identifier reaches past a route's indicated script index (as with my Viridian Glitch wall which corrupted half of the route's map makeup) it will trigger either an invalid script or one of the route's scripts.
There was discussion about encountering a trainer twice after doing the escape glitch and messing up the route somewhere on this board but I'm not sure where it was at now.
As for Torchickens, I doubt that the glitch messes with map data (right away) as NPCs are part of the map's scripting and they were all intact, minus when they pop in and out of existence. Not only that, but the BG/tileset is still unchanged. The game does something after each step that causes progressed glitchiness, but I'm not sure what. Sooner or later though, the map may change as seen in fatman's screenshots, but only after a bit of walking happens.
This info still doesn't explain Paco81's Hooked Dragonite glitch because his doesn't involve returning to the route.
I may look into this later.