Is it very uncommon for a benzodiazepine to trigger mania/hypomania?

No.: Benzodiazepines tend to be calming, but excessive use can actually cause a manic or hypomanic state.

Yes: Many things can trigger a manic / hypomanic episode but generally not benzodiazepines except in a few situations. Some can effect the metabolism of other meds, over sedation causing sleep apnea, withdrawal response if tolerance developed or abusive use. More often a manic episode is developing and the benzo hasn't helped. First symptoms mania may be mistaken for anxiety and rex with benzo.