An alternative explanation I can think of is the placebo effect. It’s possible that your behaviour Y changed after changing X, because you believed behaviour Y would change. Especially as you wanted to change those behaviours in the first place.
Also, even if this was not due to placebo effect, it’s only evidence on how your mind works. Other people’s minds might work differently. (And I suspect it’s also quite weak as evidence goes, though I can’t seem to articulate why I think so. At the very least I think you’d need a very big sample size of behaviour changes, without forgetting to consider also the failed attempts at changing your behaviour.)
(Edit: I noticed that the Oxford event is already sold out, so this is kind of a moot point for me this year, but I’d be interested in the answer anyway, as this isn’t the last event like this.)
You’ve given examples of how different active EA members have found each other and come up with new projects to do together. This supports the idea that if you’re an EA activist, you should attend the conference. But do you think “rank and file” Effective Altruists whose only aspiration is to donate a portion of their income should attend as well, even if they aren’t interested in becoming more active? Is it still more effective to spend the money on the convention than to simply donate it to an effective cause? If I were to attend, I’d expect to have many interesting lectures and a few interesting conversations, but I’d be surprised if I became more involved in the community building side.