Shedding of nematocysts, or maybe just slime that coats gills and causes suffocation. I've never had it from a BTA but everytime I've moved my ritteri I've had fish losses, even when there was no visible release of slime ... it just releases invisible fish death.
I don't ever want fish in with that thing again.
BTA's... pretty small though .. although I wouldn't rule it out completely, I'd be somewhat skeptical. Hard to say.
Anytime you change things there's always the risk of something ... it just seems to work out that way. Sorry to hear about your lion.