DataDog:
I think the main motivation for the story was simply a way to kill Moses off. After all since he was gone, they had to have a way of explaining this and to also scare the locals w/ the fear of god.
Like all good stories in the Torah, there are two versions of the same ev ent, that are the same but also different.
http://en.wikipedia.org/wiki/Meribah
The Biblical text mentions two very similar episodes that both occur at a place named Meribah . The episode recounted by the Book of Exodus features the Israelites quarreling with Moses about the lack of water, and Moses rebuking the Israelites for testing Yahweh ; [8] the text states that it was on this account that the place gained the name Massah , [1] meaning testing , and the name Meribah , [1] meaning quarreling . The narrative in the Book of Exodus states that, on account of their thirst, the Israelites grumbled against Moses, [9] so Moses, in fear for his life, appeals to Yahweh; [10] the narrative continues with Yahweh telling Moses to walk ahead of the others, and strike the rock at Horeb with his rod, [11] and when Moses does this, it causes the rock to expel water. [12]