Who would actually pay them to do the attack? Their competition? You would think their competition already makes good revenue why risk their reputation to DDoS a Bitcoin casino.

I never really understood why a casino would get DDOS.

Generally the only types of profitable DDoS is usually DDoS some small pool so the difficulty goes down so the hackers can mine more coins themselves. Basically this is what happened constantly on coins like Boolberry (BBR). If 99% of people are pool mining and they DDoS every single pool then they have a good chance at solo mining the coin and reaping all the rewards.