Also, for clarity, the game doesn't put you in combat. You can still change your build and move with your out of combat movement speed.
This is specific to dungeons to prevent "Graveyard Zerging". If anyone in your party is actually "in combat", waypoints for the party are disabled. This is why you will often see people say "OOC" -- "Out Of Combat" -- so that joining players can waypoint and catch up.