Bubble Hearth refers to a paladin who escapes PvP, or any other type of combat, by using Divine Shield and then their Hearthstone — generally viewed as a cowardly act and those who use it often mocked for doing so.
Of course, this tactic must be used when necessary, such as when a roaming paladin finds himself unfairly situated against multiple foes and does not stand any chance of emerging victorious. This is more of a smart move than a cowardly one. It would even be possible to say that the individuals attacking the paladin are cowardly for only taking him/her on when en masse.
There is even a song made by players relating to this act:
I am a paladin short and stout, Here is my hammer, here is my mount, When I see a (Horde / Alliance) then hear me shout, Then I pop a bubble then hearth right out! or Then I pop a bubble and hearthstone out!)