Back to basics! Wine is what happens after the sugar in grapes is converted to alcohol with the help of yeast, through the process of fermentation. The extra, hungry yeasts without any sugar to consume will end up dying and settling to the bottom along with the rest of the lees and sediment.
If you over-pitch, or dump in too much yeast, your squadron of cells might over-accomplish its mission, thereby fermenting too fast and stripping the beer of much of its desired character. If you're aiming for esters and other complexities that arise during fermentation, you might not get them.