Hmmmm...lets recount:
After MC sold his staff at the start of the manga, the clan that bought it managed to defeat the Firedragon, which apperantly was undefeated up to that moment. Then some time later, MC and his party managed to kill it with just 3 people. And after that, using MCs 3 role tactic, some other guilds managed to kill it too, but it still seems to be something only a few groups can pull off. And now we have what seems to be a regular monster-horde event, which is different from the ones before by having a few dragons mixed in...
I doubt thats a coincidence. Seems more like the spawn of monsters in the outside world is somehow linked to Gods Dungeon. My best guess is thats either a direct thing like "For every time a monster is killed in the dungeon, a copy of it is spawned in the horde even", or the Dungeon is like a test of strenght, as in "a monster is killed on a regular basis, so a corresponding / appropriate ammount of copies will be spawned in the horde, like for every 5 times it was killed in the dungeon, an extra copy will be spawned".