Simple. The field become's quicksand, sucking in and killing all creatures who are already inside it.
Since this is purely semantics...
Most things can't burrow in solid rock, so the field is already sand for creatures to burrow. Quicksand would just drag them down. Soooo...instead of being burrowed 5 ft under, it's burrowed 20 ft under? It's exists underground in the sand either way. It wouldn't kill it. Further, quicksand is largely a surface level phenomena, so burrowed creatures may not even feel it.
Also, since they are burrowed in the sand, it's questionable if an earthquake would kill them either, since sand is a poor transfer medium for an earthquake.
[EDIT:]
That, or only the earthquake card has this effect. Doesn't really matter. There should just be a way to counter burrowed creatures other than shields and flooding. Earthquake is the obvious answer without adding another card to the game.
But then an unupped card has an ability greater than its upped counterpart, based solely on the name of the card.
