I would bet it's stress from the barbs nipping. Add some current that they may not be usd to on top of the stress from being chased & nipped, and the fish goes more or less into shock. What type of filtration are you running? I have box and sponge filters on my growout tanks, I keep them to about 3 bubbles per second until most of the spawn is at least pea body size. Once they get a little bigger I'll turn it up little by little.
I've never seen any shop sell angels smaller than dime body size, by the time they are that big they should be able to handle plenty of current, though they don't really like it & will try to avoid it. Depending on how they were raised they may not have been exposed to much current. This is something some breeders don't think about, the tank their fish may end up in. As my angels start getting towards dime size the air flow gets opened up, think of a small pot of water on a good rolling boil. That's what it's like above the filters. The end customer may have a tank with a huge hob or canister filter, one blast from that & the angel is a goner if it's not used to it.
Turn down the air flow to the filter, and seperate the barbs from the angels. Every few days turn up the air flow a little more.
