Adam Clarke on Exodus 24:14

Tarry ye here for us - Probably Moses did not know that he was to continue so long on the mount, nor is it likely that the elders tarried the whole forty days where they were: they doubtless, after waiting some considerable time, returned to the camp; and their return is supposed to have been the grand cause why the Israelites made the golden calf, as they probably reported that Moses was lost.

Aaron and Hur are with you - Not knowing how long he might be detained on the mount, and knowing that many cases might occur which would require the interference of the chief magistrate, Moses constituted them regents of the people during the time he should be absent.