So Postumus was praefectus castrorum (camp prefect), which means he wasn’t actually the commander of the second legion. It’s not clear why he was the one making the decision. Many historians speculate that the legate and elements of the legion had accompanied Paulinus on his conquest of Anglesey, leaving Postumus in charge when Boudica’s rebellion broke out.

The praefectus castrorum was usually a veteran centurion promoted after a long and honorable military career. For this reason, it seems unlikely that Postumus simply panicked or was overwhelmed by cowardice when Paulinus called for support.

Instead, Postumus may have felt that the rebels were too strong and that Paulinus was making a mistake in attacking them without sending for reinforcements from the mainland. The rebels had already defeated the Legio IX (Hispania) and if Paulinus had been similarly crushed, then Postumus might have been vindicated in his decision to refuse combat.

Alternatively, it’s possible that the situation in southwest Britain (where the legion was based) might have been severe enough that Postumus didn’t feel the legion could move without inviting disaster. In ‘A History Of Roman Britain’ Peter Salway writes that his “disobedience probably pinned down some of the most dangerous tribes, but this does not seem to have been taken into account subsequently.”

Perhaps Postumus thought that the departure of the legion would allow the western tribes to rise up in imitation of Boudica, who had just massacred the Roman population of Camulodunum. I think a combination of those two reasons is plausible, with Postumus believing that the rebels were too strong for the forces in Britain and that he would be better off locking down the southwest until reinforcements arrived from Gaul.

But that’s all speculation. The only real information we have about Postumus comes from Tacitus, who doesn’t explain his motives, simply that “Poenius Postumus too, camp-prefect of the second legion, when he knew of the success of the men of the fourteenth and twentieth, feeling that he had cheated his legion out of like glory, and had contrary to all military usage disregarded the general's orders, threw himself on his sword.”