I think the environment aboard the first Death Star encouraged a lot of competition between officers and backbiting, including the occasional poke or swipe at Darth Vader since he was probably seen by more officers than just Motti as an unwanted intrusion by the Emperor over the day-to-day operations of the battle station. In retrospect Grand Moff Tarkin was likely one of the three or four most powerful and feared people in the entire Empire and was allowed a significant amount of leeway by the Emperor to behave the way he wanted so long as the Death Star was completed and fully operational.
Normally an Imperial officer berating Vader or even just failing to successfully execute one of his orders could get themselves killed (both Admiral Ozzel and Captain Needa learned this - and fatally so - in the next film), but Tarkin was the supreme authority on the Death Star and his inner circle of high-ranking officers were protected by him because he needed their skills and ruthlessness to operate the station at peak efficiency (otherwise he wouldn't have stepped in and directly ordered Vader to stop choking Motti) and Vader was not at the top of that localized chain of command. Hence, some officers felt they could get away with expressing themselves more openly and honestly than if they had been serving aboard Vader's own Star Destroyer.
Normally an Imperial officer berating Vader or even just failing to successfully execute one of his orders could get themselves killed (both Admiral Ozzel and Captain Needa learned this - and fatally so - in the next film), but Tarkin was the supreme authority on the Death Star and his inner circle of high-ranking officers were protected by him because he needed their skills and ruthlessness to operate the station at peak efficiency (otherwise he wouldn't have stepped in and directly ordered Vader to stop choking Motti) and Vader was not at the top of that localized chain of command. Hence, some officers felt they could get away with expressing themselves more openly and honestly than if they had been serving aboard Vader's own Star Destroyer.