I think one of ways to trigger a bonus is not to play for few months and then log in. Once I got "Welcome back" bonus with around 10 USD worth of their casino token (i think BCD). Also wagering requirement was 35 X so I spent it way before I wagered funds. Basically you can't get those funds out, maybe if you are extremely lucky.
IIRC some user in the past mentioned that they received this welcome back bonus even though they are actively playing in the casino. Maybe this is just a bug but I read that multiple user claimed to experience the same.
So far, I don’t experience anymore getting welcome back bonus after a long time of inactivity.
I think the previous wager before you become inactive is also another criteria for this bonus.