"His fractious temper coloured all the controversies in which he took part, in Gaul, Africa, and Italy, including Rome, where at his death the clergy were very much divided." ("Pope Zosimus," Wikipedia)
"He tried to expand the power of the pontificate, but allowed his personality, and his personality clashes with local bishops, to enter into the matter." ("Pope Saint Zosimus," Saints.SQPN.Com)
Zosimus seems to have irritated every bishop and most priests in Gaul by elevating the the Bishop of Arles to Metropolitan, effectively making him a layer of management between Rome and the other bishops. It might have aligned well with the Roman civil government, which was probably important since the Imperial Government had just driven the previous Bishop of Arles into African exile, but it hardly sat well with the ecclesiastical crowd. As the quotes indicate, he was diplomatically clumsy, even when he was attempting to tread gently.
Further, issued an order banning clergy from entering taverns. We're not talking brothels here. Just taverns.
So why exactly was this guy a saint?
It is also, of course, Boxing Day, the Feast of Saint Stephen. Stephen was one of the first seven deacons and the very first martyr, stoned to death for his faith and works. One might appropriately play "Good King Wenceslas" today, just to take the edge of the cold turkey withdrawal from Christmas carols. Not me though: I'll take Elvis Costello's "St. Stephen's Day Murders," performed here by Thea Gilmore.