For those of you that really need to know the answer:
The Hivers did it.
We manipulated the interstellar situation so that Strephon was killed.
As we said at the time: the 3I was far enough away from the HF so that there would be no local difficulties and we felt (and still do feel) that the Imperium was rotten to the core, so all we really did was give it a little push. It was inevitable that the 3I would collapse in some fashion, either under its own weight, from internal factors or from external factors.
However, despite the extensive use of psychohistory (you didn't really think we'd ever stopped using psychohistory, did you?) we did not examine the effects that one person could have on the overall outcome. M. Hari Seldon tried to warn us about this; we ignored him. We ignored him because his theory could not be backed up by the psychohistorical data we had amassed on the 3I.
Equally, being focused on causing the downfall of the 3I, we neglected to examine the possibilities posed by the K'kree. So secure were we that the K'kree had learned their lesson during the Hiver-K'kree war, we did not consider them a realistic threat.
Humaniti paid the price for our ignorance.
"But Gruffty," you say, "You weren't there during the Rebellion, Hard Times, the Collapse. Where were you, Gruffty, when the lights went out across Humaniti's worlds?"
Sit down, children, and I shall tell you my stories...