system
Given the following post, respond as if you're commenting on Reddit:
human
[Star Wars] Why did the Y-wings die after a single hit from the TIE fighters in the trench?

Aren't Y-wings supposed to be well shielded and take a lot of punishment?
gpt
A combination of a number of things: 

* Y-Wings were notoriously hard to repair. Maintenance crews had to basically take the entire ship apart to put in new hull plating on anywhere but the cockpit meaning that they were exposed from behind once their shields were down. (Importantly, they were getting hit *only* from behind on the trench run.)

* On that point, Gold Squadron only had a few days between the battle of Scarif and the assault on the Death Star. They were flying ships that were already battle damaged with no time for a full repair.

* Thirdly, and maybe most importantly, the Y-Wings were not doing what they were built for. They were slow, lumbering ships that were equipped for a 3D space battle. Putting them in a gravitational flight, in a small enclosed trench where their enemies were directly behind them was the worst possible scenario. One good hit and they could easily crash into either side of the wall or the floor. (Notice how when Wedge's X-Wing is hit, he still has enough control to pull out of the trench.)

Basically, they were the wrong tool for the job but at that point the Rebellion had to throw everything they had at the problem.