Issue is thus:
Client creates a single connection to MongoS
Client issues a query with read preference "Secondary" and read preference tag of "a"
MongoS creates a new connection to secondaries tagged "a" and issues queries
Client issues a query with read preference "Secondary" and read preference tag of "b"
MongoS dropps connection to secondaries tagged "a" and opens connections to secondaries tagged "b" then issues queries
Client issues a query with read preference "Secondary" and read preference tag of "a"
MongoS dropps connection to secondaries tagged "b" and opens connections to secondaries tagged "a" then issues queries

If the client continues to change read preference tags then connections will be opened and closed following each query issued on that connection with a new tag. This can lead to large amounts of connection churn.