I think the flag was thrown because it sorta looked like Offensive Pass Interference. This is when the receiver interferes with the defender being able to make an interception.
However, on the replay it seems like to me both players were playing the ball... that is they were going trying to make a catch on the ball. I think it was the right call to pick up the flag. Now, I would have loved to see the call stand because it would have helped the Lions, but I don't think there was interference there.
There was a play earlier in the game where I thought there was definitely OPI, but not on this one.