I would say that the reason why a given organism reaches a given space relies on four main factors:
- Morphology and physiology: can that organism reach that place physically?
- Food distribution: is there enough food to reproduce and spread in that specific spot?
- Habitat: does that habitat fulfill its environmental/physiological needs? Is it environmentally safe for that species?
- Abiotic factors: wind, water, etc...
I know it's a very general answer, but consider that the main goal of every organism seems to be to survive, spread and reproduce as much as we possibly can.
For example, if the gutter accumulates moss, earthworms will most likely crawl up the wall and into the gutter to feed on the precious organic matter. Also, gutters, due to its function, tend to be wet at times, and earthworms need a moist environment, so it seems like an ideal spot for them (earthworm climbing up).
Live worm dropped by an overflying bird
Seems likely too: the term ornithochory defines seed dispersal by birds. I guess it could also be applied to dispersal of other animals (at least anthropochory can be applied to both seeds and reproductively capable animals dispersal by humans, in this case, earthworms being transported by the bird's beak). I think the most likely scenario in this case is food availability and a good environment to proliferate and make more earthworms (mostly because it would take a couple of clumsy birds to drop their prey on top of your gutter to establish an earthworm colony up there although it depends on the specific earthworm species and whether it is parthenogenetic or not, of course, but still within the realms of possibility).
When it comes to species distribution, it normally tends to be a mixing of different mechanisms rather than one single specific mechanism.