The thing is, grumbling and complaining doesn't usually start big. I'm betting that they didn't go from "Yeah, God is giving us food from Heaven" to "Manna again!?!?!? I'd rather eat me shoes than eat mana one more time" overnight. No it started small. Someone woke up and said "manna is nice, but I gotta say I'd like some meat to go with it." He said that to himself a few times before he said it to someone else. Now he could have gone to Moses or whoever was setup over him in the system Moses setup but he started talking to someone else. That person was pretty happy to be eating food God setup for him every morning but he politely listened and maybe even tried to offer some words of advice and encouragement. After a few days of hearing the complaints he got tired of it but also started thinking that the manna was starting to taste a bit bland. Suddenly a virus of complaints took over and everyone was infected with dissatisfaction.