I'm by no means an expert, but I wouldn't think it would be excess yeast; you start with (relatively) very little yeast and it multiplies to eat the sugars in the wort as needed.
I would guess lowering the malts (I'm assuming it isn't just cane or corn sugar) becomes a question of balance between the sweetness of the malt and the bitterness from the hops along with avoiding a "watery" taste. Keep in mind, most of the "low alcohol" beers I've seen homebrew recipes for are not like the LA beers you buy, but are just less than normal, hitting an ABV of 2.5%-3%. For example, I'm thinking of brewing a mild brown that BeerSmith has estimated at 2.99% ABV, but that's nothing like an O'Douls "no alcohol" beer, nor would it be considered by most to be a "low alcohol" beer. I did see one article about removing alcohol after the fermentation by heating in the stove, then (if not kegging) adding back yeast for bottle conditioning; it's another option.
I would use BeerSmith to play around with the numbers while watching the IBUs and bitterness ratio to keep it in balance, but I'm just guessing as I only have three batches brewed and none a low alcohol beer. You might end up with beer-flavored water.
Rick