How does the cleric enchants work?

I have noticed a few times that my items suddenly got enchants and I assumed that maybe it's soemthing that happens over time. Then in my latest game I got a vmpiric sickle which I was very happy about, until I equipped it and saw that my vampiric enchant got overridden by the holy enchant. A few floors later, I see that my sickle has gone to vampiric. Now I'm very confused and I couldn't find info about I on the wiki