Executor crashed into DS II because the bridge got exploded and it was left without controls.
Without control, it got dragged towards the DS II's gravity field. And if you want to argue,
How is everyone not floating around on the DS II then?
Ackbar knew that the modular KDY bridge was poorly defended and ordered his ships to specifically target it.
The shields were basically down when the A wing went through it.
Also, there are two different types of shields in SW.
Particle shields (for physical objects) and ray shields (for lasers and energy weapons).
Particle shields are typically weaker than ray shields, because stuff like concussion missiles and proton torpedoes are expensive and prohibited by the Empire.
The HVs-2 hypervelocity gun, or simply hypervelocity gun, or sometimes referred as an electromagnetic gun, was a fixed planetary defense weapon used against orbiting space-craft such as capital ships by the Galactic Empire during the Galactic Civil War. The hypervelocity gun fired bursts of...
starwars.fandom.com
"The gun's slugs would penetrate a ship's ray shielding and quickly batter down
its particle shields intended to absorb slower and fewer impacts. Once a ship's shields were down, the slugs would batter its armor, eventually rupturing its hull and causing depressurization."
Sounds like a certain fried ice cream cone out of the Maw Installation that came with special armor, stronger than any ship of its size and smaller.