Has it been established that there was a temple flock year round? The established practice in that area was to move the flocks from pasture to pasture in nomadic fashion. Given that there were buyers and sellers (according to one of my theology professors in the gallery of the gentiles) it appears that the animals used for sacrifices weren’t kept by the temple but were brought there by merchants who sold them to individuals following the law.
the shepherds were part of the lowest memebers of society. the merchants were somewhat higher. It would make sense that the merchants hired them to tend those same flocks.