If there are multiple monsters tied for the highest ATK, they are all destroyed.
If “Bottomless Shifting Sand” is active and “Tyrant Dragon” is the monster with the highest ATK, “Tyrant Dragon” is destroyed because “Bottomless Shifting Sand” does not target a monster.
Netrep Rulings:
If more than one monster ties for highest ATK value in the End Phase while “Bottomless Shifting Sand” is active, all of those monsters will be destroyed by the effect of “Bottomless Shifting Sand.