I left the part out about how we were about to round A mark: so I guess this would apply:

18.3 Tacking When Approaching a Mark
If two boats were approaching a mark on opposite tacks and one of
them changes tack, and as a result is subject to rule 13 in the zone
when the other is fetching the mark, rule 18.2 does not thereafter
apply. The boat that changed tack
(a) shall not cause the other boat to sail above close-hauled to
avoid her or prevent the other boat from passing the mark on
the required side, and
(b) shall give mark-room if the other boat becomes overlapped
inside her.

since the boat that tacked in front of me caused me to sail above a close hauled course, they fouled me.