There is a catch, but you won't find out what it was until they loose all of the money they are holding. There is probably a 50/50% chance this is just a Ponzi, and if it isn't it is some complicated strategy which can not handle black or even grey swan events, and there is a 5% chance there is a bug in their code that allows for all the funds to be stolen.
But in the mean time you can get an 8% return.