Good point about the proximity, BU. It can be hard enough not to be jealous of your partner's primary at times even with sex AND love AND compersion, just because they'll always be "senior" to you, like an older sibling situation but more emotionally charged, and because it can hard to see and to know about special moments, not all of which you can possibly share.
One really bizarre trigger for me is around their anniversary. For the past two years, though I've managed to control it, I've gotten weird and jealous around that day. No good reason for it at all.
I think people really underestimate how much work secondaries have to do to stay emotionally healthy...