Dwarfs will drop items related to a job when startled by wildlife. Plant gatherers will carry many plants at once (as will haulers who are filling a food barrel). Are you seeing any "... interrupted by..." announcements?
That could explain the outdoor multi-stack pile(s), if the herbalist was holding a bunch of different stacks. But I don't remember seeing interruption announcements. If those can be turned off, I might have them off.
Also, that doesn't explain when I see a trail of single stacks of plants coming from a food stockpile inside my fort.
They're not plants taken out from barrels or bags, they're plants not yet stored inside them. Herbalists don't always haul back their produce(don't recall cause off the top of my head but it also occurs with map wall off to top of the sky), make sure you have decently sized empty stockpile area to collect these - if you just have a few tiles, some of them are going to wither.
That would make sense to me if I was seeing many individual piles of single stacks of plants. Right now, though, I have one tile with a page and half of variously-sized stacks of different plants. If herbalists were dropping after finishing a job, wouldn't they just drop the plant right where they finished gathering it? Or is one herbalist gathering plants on many different tiles, holding all the results, and then dropping all the stacks when they switch away from the gather-plants job?
I guess fundamentally I don't know how herbalists gather. I have some plants designated for gathering with d->p, and I have some plant-gathering zones with shrubs enabled. I don't know how many stacks of plants a dwarf can hold, and I don't know how they go about actually gathering the plants. It would be interesting to understand how exactly this works.