Players can be sensitive (and even assholes) to draft picks who are there to eventually take over (and let's be honest: eventually, Simpson is going to be our QB1, otherwise, we wouldn't have taken him top fifteen). Look no further than Marshall Faulk's entire relationship with S-Jax (although S-Jax certainly didn't help matters at first with his own brashness. To the credit of S-Jax, when he was being replaced by Isaiah Pead and Daryl Richardson in 2012, both far inferior backs to him, and knowing it was his last year with the Rams, he taught them as much as he could.). Favre and Rodgers are also notable examples who were sensitive (or rather, utterly fucking butthurt jerkasses) to their replacements.
My post wasn't meant as an indictment against Stafford, but a positive attribute towards Stafford and appreciative of what he's doing and how he's going about it, especially since it's the Super Bowl favorite team, and because we drafted another quarterback with Atlanta's top-fifteen pick instead of someone who could "help immediately" or trade down. It's not easy, and some players have issues with their replacements and treat them poorly when they're still starters.