Good question...I’m a naval person , but I would think blowing bridges is a defensive or a retreat blocking operation.
Blowing bridges is both an offensive and defensive strategy depending on the motives of the side doing the blowing up.
Offensive…. Blow a bridge to keep the opponent from receiving supplies and reinforcements or to channel the opponent to ground that is disadvantaged
Defensive…. Blow a bridge to cover a retreat or defend from attack from an undesired direction.
More and better examples are out there I’m sure.