“Why do red worms, not all but about 15 or so stay on the top inside of the bin cover. The container is a can o worms, It is not continual but frequently.” ~ Jean
That seems to be one of the mysteries of the universe – why are some worms more than happy to stay down, while others insist on roaming. I think that, like humans, some worms just happen to be more curious/adventurous (in a very limited sense) by nature.
Generally, I don’t worry too much if only a handful of worms are up on the sides and lid, but of course if you start seeing more of them, and certainly if any of them are balled up together there may actually be something wrong. I’m sure some simply encounter the sides of the bin and then just follow it up, since there is no where else to go.
I actually like to keep a thick layer of dry bedding at the top of my enclosed plastic bins since this seems to work well for keeping conditions fairly dry up above, and encouraging the worms to stay down. Alternatively, keeping the lid off altogether can work quite well too – although with a stacking system this might make it difficult to keep everything moist inside.
Anyway – I realize I haven’t really provided you with a good explanation here, but hopefully the fact that I am as clueless as you are at least provides some peace of mind!