What was it Yaakov's business if the shepherds were idling when they shouldn't have been?
Rashi: Because if they were hired shepherds, they were wasting their employer's time (which is akin to stealing); whereas if they were self-employed, it was not time to take the sheep away from the pasture, in which they were depriving the sheep both of their food and by not watering them there and then. 1
Ohr ha'Chayim: Tza'ar Ba'alei Chayim is Min ha'Torah (Bava Metzia 32b).
Ha'amek Davar: He wanted them to go away so he could talk alone with Rachel. 4
Gur Aryeh: Thus, the verse can be divided into two separate points - If you are hired shepherds, ''the day is yet long" i.e. your work-day is not yet done. Even if you are the owners, it is still "not time to gather the flocks."
Seforno: As the Pasuk writes, "A man of iniquity is the abomination of the righteous" (Mishlei 28:26).
And because he was sincere in his rebuke, the shepherds accepted it and did not accuse him of not minding his own business. (EC)
He cites this from Midrash Rabah, but I did not find it. It would not be seclusion if they went away, for people are prone to come at any time to the well. (PF)
Why did he say "Mikneh," and not Tzon?
Ha'amek Davar: One who has few animals can take them to his Chatzer and feed them there. Mikneh implies a large flock; they need to graze in pasture!