Can you please provide more contexts to your sentence?
The phrase "milk run" originates in American culture, with the
distribution of milk bottles by the milkman. On his daily route, the
milkman simultaneously distributes the full bottles and collects the
empty bottles. After the completion of round trip, he returned with
the empties back to the starting point.
On the round trips are either goods collected from several suppliers
and transported to one customer, or goods collected from one supplier
an transported to several customers. In contrast to the groupage
traffic, there is no handling, except to transport the goods.
I’m speculating here.
The tour operators are taking/busing the tourists to their destinations. While dropping off this group of visitors at A, the operators are picking up another group of travelers and taking them to destination B.