It looks like the homeless population has remained fairly steady over the same period of time that reported poop incidents has quintupled. That leads to two likely conclusions: Either people are reporting it more often than before (Has it become easier to report such incidents?) or more homeless are deciding (as much as it can be called a decision) to poop publicly, maybe a further symbol of spite for a society that has left them behind?
I suppose there's also the possibility that homeless have been under counted, but to do so by to such a degree seems unlikely.
I would imagine that it's a mix. A rise in reports (thanks to the availability of the 311 mobile app and such), as well as a rise in incidents. I don't have much to back up the rise in incidents (besides everyone's general feeling that things have gotten worse on the homeless front, and maybe something to do with the opioid crisis), but it certainly feels that way anecdotally
As another anecdote, I've not noticed an increase in human waste over the last few years, but I have noticed an increase in discussions about reporting it.
Another possible cause of the rise (with no evidence to support) could be demographics of the homeless population.
I imagine propensity to poop in public is correlated with other behaviors that make it difficult to get off the street.
If SF is getting people off the street slightly slower than new homeless people are being added to the population, and the set of people being reintegrated are less likely than the median to poop in public, over time we'd expect to see a slowly growing homeless population that is increasingly likely to poop on the street.
There may also be a matter of changing norms; if there's already poop on the street, adding some more (vs making the trek to a public restroom that may be unsafe) isn't as unappealing.
I suppose there's also the possibility that homeless have been under counted, but to do so by to such a degree seems unlikely.