Something to note about the destruction of the Executor.
Prior to its bridge deflector shield being knocked out it was under heavy bombardment from all avalible rebel ships. Admiral Akbar had ordered all firepower to focus on the super star destroyer.
Both sensor domes on top of the tower were destroyed. This might have caused the bridge deflector to fail or was the first indication to the ship's crew that the deflector had failed. The A-wing that smashed thougth the bridge exploding somewhere inside the superstucture as the whole tower starts to blow and the underside of the ship opposite the bridge tower is also spewing fire (in fact the plume on the underside is far larger than the one from the bridge tower). That would likely mean that engineering was also hit, or the blast traveled down the tower and took out more sytems than just the bridge. Likely it was the additional firepower of the Rebel fleet into the opening the A-wing provided finished her off.
Executor, and the Imperial fleet had followed the Rebels into low orbit of the Death Star. With its systems compromised the giant super star destroyers was caught in the gravity well and pulled into it, there seems to have not been enough time for secondary systems to kick in, or perhaps the crew was trying to get orders, not knowning the bridge was gone. Whatever the case, they were unable to solve the problem in time and she impacted with the surface of the Death Star II.
I think those domes are the deflector generators and not sensor domes
[yt]https://www.youtube.com/watch?v=RW_hGOFukMQ[/yt]
We see one blown up and the very next line is the bridge deflectors have gone done.