Why does the Torah add the word "le'Mas'eihem"?
Ramban: Because they traveled from Midbar Sin to Refidim via Dafkeh and Alush (where the episode with the Manna took place). 1
The Torah omits these two stops here, because it is only concerned with the places where they grumbled - in Alush (at the beginning of Midbar Sin) they grumbled about the lack of bread, and now, when they arrived in Refidim, about the lack of water (Ramban).
What exactly happened here? What were they grumbling about?
Ramban: They arrived at Refidim and did not find water there.
Targum Yonasan: The local water-springs dried up.
Why did Hashem leave them without water?
Targum Yonasan: Because they were lax in the Mitzvos that they had already been commanded (at Marah)?