What was it Yakov'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.
Ohr ha'Chayim: Tza'ar Ba'alei Chayim is Min ha'Torah (Bava Metzi'a, 32b).
Ha'Emek Davar: He wanted them to go away so he could talk alone with Rachel. 3
Seforno: As the Pasuk writes in Mishlei (28:26) "A man of iniquity is the abomination of the righteous."
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'Emek 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!