Carl Banks was saying on the podcast this week that it's a communication issue. Like, when you're in Cover-4 the whole idea is that you're defending against the deep pass... there's no reason why corners should be biting on the 8 yard square in while somebody is running free behind them. Corners have to know what the idea behind that defense is and why you're running it, and be smart enough to do the right thing.
Also, zone defense isn't just "stay in a spot," you have to be cognizant of what's happening and who's coming into your zone and trail them to a certain degree, which the Giants defensive backs are not doing. He described it as being "man elements to zone defense."