Grignard alkene on a carbonyl
Nucleophilic acetyl substitution is common among carboxylic acid, acid chlorides, anhydrides, esters, and amides.

Two very common examples are anhydride decomposition and esterification of carboxylic acids. For example, if you look at esterification, carboxylic acid and alcohol first make an addition, giving tetrahedral intermediates 1 and 2. Intermediate 2 then collapses and displaces the water molecule as a leaving group to give the corresponding ester (see the scheme).



Your substrate is an amide. Like all other carboxylic acid derivatives, this molecule also undergoes a nucleophilic acetyl substitution with Grignard reagent, which is a strong nucleophile. The polarized carbonyl group has partially positive carbon, which is a good electrophile.

The nucleophilic carbon on Grignard reagent attaches to carbonyl carbon and gives tetrahedral intermediate as a metal alkoxide complex. But, this alkoxide can collapse to reform strong C=O bond, displacing the amino portion of the amide as a leaving group, in the form of the amide ion, $\ce{R1R2N-}$. This produces the ketone as a final product, assuming you have used only one equivalent of Grignard reagent (Channel reactions of Grignard reagent with esters).


  Why does the reaction continue after the tetrahedral carbon forms?


After nucleophilic attack on a carbonyl, the resulting tetrahedral species either gets protonated to form the final addition product, or it is an intermediate on the way to the substitution product. This depends on the type of reaction. For example, a geminal diol will usually dehydrate to form a ketone. There is a nice overview of the different scenarios, including some nitrogen chemistry, in this document.