In grappling competitions, a sandbagger is known as somebody who fights down his/her normal skill level in order to get easy wins.
For example: A grappler who has an intermediate skill level (2-4 years experience) will decide to drop down to fight in the novice or beginners divisions in order to sweep everyone out of the division and win the gold.
Wrestlers are usually the ones that sandbag, but other grapplers do it as well.
Regardless though, sandbaggers are pussies and should be beat for their bad deeds. :P