I don't know anything about anglers, so here's some speculation:
I would guess that it's feeding-related. They eat lots of big food at once, and live food at that (iirc). I don't know what's used, but overfeeding is the [proposed] leading cause of lionfish death and underfeeding is a big killer of certain eels too (namely ribbons). I can't think of what you would even feed a full-grown angler as they eat such big pieces.