The Championship side have picked up eight red cards so far this season, including five in their last two games.
Thompson told Sky Sports News: "I don't think it's a lack of discipline. It might be in part but it is also down to a lack of judgement.
"It comes down top self control. When you are on a yellow card, you are walking as tightrope and the last thing you need to do is make a rash tackle. That's what's letting us down at the moment and it has to stop happening.
"It can be down to the discretion of the referee too. The linesman gave the penalty for the first red card but Bishop seemed to come out with ball, which suggests he got the ball."