It seems to happen upon being hit by the mage guild ability, ice comet, my character falls over and as it gets up it falls through the overlook of the fort (above the main gate and I die to fall damage which gets extremely annoying
If so, don't stand on the metal grates - knockback abilities (Meteor, Flame Clench etc.) will knock you down through them. People know this and deliberately target those who do wander onto the grates.