When the defending bishop cannot influence the square in front of the pawn, the attacker pushes with check and uses bishop checks to drive the king away from the promotion square.
The black king stands on e8, next to the queening square d8, and the defender might feel safe. The problem is geometry: d7 is a light square and both bishops are dark-squared, so neither bishop can ever contest d7. The pawn advances there for free, and the fight is purely about d8.