Abu ‘Abdillah, Muhammed bin Ebrahim Al-Kinaani mentions:

I once asked Abu Haatim Raazi (rahimahullah), “Why is the Muwatta of Imaam Maalik (rahimahullah) named ‘Muwatta’?” He replied, “It is a kitaab which Imaam Maalik (rahimahullah) prepared for people to follow (one meaning of the word Muwatta is ‘a path that is made for people to tread on’), until it became known as the Muwatta of Imaam Maalik (rahimahullah), just as the kitaab of Imaam Sufyaan (rahimahullah) was known as ‘Jaami’ (which means comprehensive).”

‘Ali bin Ahmed Al-Khaleeji (rahimahullah) reports that he heard one of the Mashaayikh say that Imaam Maalik (rahimahullah) mentioned the following, “I presented this kitaab of mine (the Muwatta) to seventy Fuqahaa from the Fuqahaa of Madinah Munawwarah, and all of them concurred with me and agreed on the knowledge that I had presented in the kitaab. Hence, I named it the Muwatta (the word Muwatta comes from the word “Waata’a” which means ‘to concur and agree’).”

(Tazyeen-un-Mamaalik pg. 89)