As of server 2.8 and server 2.6.4, mongoS no longer pins secondary connections to allow changes in read preferences between queries.

Special attention, from ticket: "As these changes could not be backported to 2.6, a different fix was implemented specifically for 2.6: a new mongos server parameter, internalDBClientRSReselectNodePercentage was introduced. This can be set to any value from 0 to 100 (defaults to 0) and represents the probability (expressed in percentage) of a replica set connection in mongos to reevaluate replica set node selection from scratch, regardless of the compatibility of the current read preference to the last-used secondary node. Extra care should be taken since reselecting a replica set node will destroy the old connection and create a new connection. This means in extreme cases (for example, 100%), mongos can be creating and destroying connections for every read request."

Text re: pinning at http://docs.mongodb.org/manual/core/read-preference-mechanics/ should be changed.

See linked.