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I've had issues with emails being put in spam because they've been forwarded. The one that comes to mind is dropbox emails being marked as spam because they were forwarded from an MS Exchange server to my gmail. Gmail reckoned it was a phishing attempt. It's probably some critical part of the header being stripped out/not recreated in the forwarded email. I would have thought it's more an issue for the person/service doing the forwarding than for the original sender.
Edit: sorry @Dammit, that should have been a reply to @TheShipwright.
hamrack-
Depends on what domain is appearing in the 5322 for a DMARC based answer to what's going on, many (understatement alert) reputational factors will be involved by the larger ISP's, also, which if you are shit at authentication (as Dropbox are showing the world that they are with p=quarantine) becomes very important.
Dammit
It looks like you're already having policy (what the DMARC record says to do) over-ridden by your ISP as they should (if it's a double failure) be deleting the email rather than putting it in the spam folder.
It might (outside chance) be being spam-foldered due to failing SPF, but not being deleted because it passes DKIM, but the sending IP reputation should be good enough to make that a non-issue.
Look in the header and see what it's passing and what it's failing and report back.