There's one thing that's certain with any new MMO launch, and it's the fact that players will always find clever ways of exploiting the unintended. In this instance, and is the case with WoW Classic, it turns out there was a bit of broken code when it came to raid groups entering dungeons. Effectively, they didn't suffer XP loss and instead, could far it freely as a 10 man (instead of 5). Needless to say this not only allowed all the dungeons to be incredibly easy, but resulted in players only staying in dungeons. 

Most streamers did this, a few racing to 60, and it seems most players in towns once they caught on to the fact it was a steamroll. Fortunately, Blizzard have stepped in quickly and have squashed the exploit, resulting in its removal. 

Thanks to player reports and feedback, we’ve identified a bug affecting how XP was calculated for parties of 6 or more players.

We are rolling out a hotfix right now, to all realms, that should correct this. Going forward, XP awarded should be significantly lower for parties of 6 or more players.

Again, thank you very much for your feedback.