Roethlisberger actually has historically made poor decisions. Last season for example, the Super Bowl season, he threw interceptions at the rate of 1 every 25 attempts. In comparison Brees last season threw them at the rate of 1 every 35 attempts. A significant difference.
The reason Roethlisberger never acquired this reputation before is they simply didn't throw the ball a lot. He had only 250 attempts last season to Brees' 500 attempts. But in the Super Bowl if you remember he played pretty poorly.
This season they are relying on him more then ever before. He's throwing 30+ times a game for the first time in his career....and it shows. (he has almost as many attempts this season as he had all of last season). He seems to be playing worse then before, but I think a lot of that is attributable to the fact that he doesn't have the field position support (from his defense) or run game support he had in the past.
Honestly, Roethlisberger just makes poor decisions and he always has. It's just never mattered until now.