I think drops are contagious through a team. Amari started it and then it went to Cobb and Gallup. I think a bit of it is on Dak as well because the receivers really don't know where to expect the ball and get frustrated.
I'd have to say that it's a combination of factors, including to some extent, the quarterback. Amari has a history of some inexplicable drops throughout his career, and Gallup clearly has a history as well. But with Cobb, I believe one year he had the fewest drops in the league, and then after arriving here, they're way up. A disturbing pattern and frustrating trend. This offense would have been so much more effective without that large number of drops.
It's a mental thing and gets worse the more you dwell on it.
For Dez, it was a huge mental thing. It's funny because the video Aviano90 posted was an atypical drop where the receiver was late getting his hands up because of hand fighting and he had to turn and jump for the ball. It is a drop but not the kind I get irritated with.
I don't make any excuses for it, Dez should have caught that one. And as you mentioned, a lot of it is a mental thing, and Dez was not in a good state of mind at the time, or after he was cut, he admits that himself. He's clearly an emotional guy and he allowed those emotions to get the better of him. In some ways, his passion is a great thing, in other ways a terrible thing.
He says he has a better handle on them, and I would give him the chance to prove it. If not, he's gone.