In this context a "SandBagger" is someone with Master-level skills who refuses to cross the promotion line to Master... thereby being allowed to play with Master-like skills from the Pro tees.
Which in and of itself is not a bad thing, until you consider that the 'SandBagger" is competing for credits with an unfair advantage.
If the alleged "SandBagger" never played a game where he/she could win credits or prizes it really wouldn't matter....
I hope this clears things up for you.