That is exceptional. Crazier yet when considering most of the orcas that travel in small groups (1 or 2-3) are usually "transients" which feed mostly on marine mammals rather than salmon like the Southern resident pod orcas (larger groups) which are salmon feeders.
Maybe a seal grabbed the fish and then the orca grabbed the seal while it was trying to snack. Or maybe that orca just decided that the struggling salmon looked tasty.
Whatever the scenario that is an awesome experience.
That is crazy!! That will be one of those moments you'll remember for the rest of your life like it just happened. I've heard of that happening from a boat, with gear rods, but never from the shore and with a fly! Wow