well. the reason why ketone's are acidic in the first place is the alpha hydrogen (the H on the carbon adjacent to the carbonyl group). however, since you have a dicarboxylic acid (such as oxalic acid, malonic acid, succinic acid, glutamic acid, adipic acid, or pimelic acids.. etc.) the alpha hydrogens are going to be DOUBLY acidic.
think of HOOC-CH2-COOH which is malonic acid, the COOH groups are naturally e- sinks in which it withdraws electrons from the center (bolded) carbon. And since there are two COOH's, the electrons are pulled away in two opposite directions, therefore, barely any electrons are helping hold the H's onto the carbon! Malonic acid will give up those protons easily. Its a doubly-alpha carbon with two acidic protons! (Aside from the two terminal H+'s on the COOH groups) =]
