Some small breeds get them sometimes more than other breeds.
It can be something like entropion.
It can be something in the duct itself.
Or like my little weirdo-there can be a lack of a tear duct completely-which we found out when the vet put her under to do a second clearing of the duct with a smaller instrument. So she always has this leakage:
I am sure there are other things, but I obviously would recommend seeing a vet!