There is an invisible "pan switch" which links and unlinks pairs of busses (this also depends on the linked pan mode, see below).

Pressing and holding the SHIFT key while pressing one of the ASSIGN keys operates a split or join function on a buss pair, allowing busses to be individually assigned when split.
(DM3200 User's manual, p.52)


Even if the busses are linked, the panning to busses can be overrode by having the "BUSS PAN Follows ST PAN" preference, unchecked. This can be found on the OPTIONS>PREFERENCES page.
(DM3200 User's manual, p.35, Figure 2.32)