What are the relations between analog transmission frequency and multipair cable types?
One thing I can think of is, if Pair #1 happens to have a large common mode voltage present, without individual shields this voltage can capacitively couple that voltage onto an adjacent pair (Pair #2 in sketch) as a common mode voltage. 

The cable spec should show a pair-pair capacitance per unit length.  Knowing the total lumped capacitance and frequency of the common mode voltage, and the impedance of the pairs with respect to ground, you should be able to calculate the magnitude of voltage that is coupled to the adjacent pair without shielding.