Slimeballs on the surface?
According to the wiki, Slimes will spawn:
Below level 40 in specific chunks, any light level
Swamp biomes on levels 51 through 69, light level < 8
There's also a section containing more details of Slime spawning but the most likely answer could be the biome.
It's common for the smallest version of the slimes to be unable to get out of the water in swamps, and drown, leaving slimeballs all over the place.
The slimes that spawned there were most likely the smallest version and had drowned. I have seen this before too.