I had 2 Angelfish in my "river tank" and thought everything was fine, then I started to notice fins being ragged only slightly then a few weeks later noticed actual chunks being taken out of one and also on another fish, never actually saw the fighting though just general chasing hear and there. Angelfish can be very aggressive when no one is looking...lol. I finally decided it was to much for just the 2 to stay together so after QT the hurt one I added 3 baby angels to the 2 I already had and I have not since found any fins being nipped or wound marks. Fish can still get stressed out and just get chased to death to the point they die (I honestly think they can have heart attacks to) or get caught trying to get away like it sounds your might have done. Usually if one dies from a parasite or disease it will start to show in the other fish, but since you don't have any other fish sick, there's a good chance it was aggression, especially if you didn't buy them as a matted pair, I would assume they might both be males, very hard to tell the difference depending on the angelfish species you purchased.