I was clear astern on starboard downwind and we collided — how is the foul mine?

Under Rule 12, when you're on the same tack and not overlapped, the boat clear astern keeps clear. Sit so close that your wind-shadow blankets the boat ahead and forces her into an unintended gybe and contact, and the foul is yours — even though you never left starboard. The boat you compelled into the breach is exonerated under Rule 43. (Rule 19 / obstruction was argued but did not apply — the boats were never overlapped.) World Sailing Case 30.

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<p><a href="https://sailing.kiwi.nz/run/case-30-clear-astern-keeps-clear/">Source: Sailing Kiwi</a></p>