Trigger Effect

Trigger Monster Effects activate whenever a certain condition on the field, the nature of play, or the progress of the turn, is met.

Zaborg the Thunder Monarch's effect is triggered when it is Tribute Summoned, and it the effect requires it to destroy 1 monster on the field. Since this is a Compulsory effect, if Zaborg the Thunder Monarch is the only monster on the field, it must destroy itself by it's own effect.