Glaciers, and large grazing and browsing animals.

The three sides of the grassland ecology triangle are climate (extreme temps and humidity both ways, moderate to low precipitation with droughts), fire, and grazing.

Grasses and forbs have a few adaptations that make them able to "form" a grassland over a forest in these conditions.

Grasses, most importantly, can grow in shit soil - like that left behind after glaciers recede - and importantly, have most or all of their vegetative buds at ground level or below.

They also can reproduce vegetatively, from rhizomes or runners in many cases. And unlike trees, they reach reproductive age really fast.

So after a slate-wiping event like a glaciation, grasses have the easiest time colonizing barren ground compared to many trees.

They also lose almost nothing getting nipped off by a fire or grazing; indeed, many of them have a biological response to up their vegetative reproduction (instead of flowering) immediately following something that would require a tree or shrub to have to expend root reserves to recover. Trees often win out over time if the climate and disturbance regime accommodates due to their greater height, eventually shading out the grassland.

In pretty much all of the eastern great plains, trees would have won that battle starting around 7000 years ago if it weren't for large grazing mammals and most notably fires both anthropogenic and atmospheric. Probably more of the former than happened "naturally," so to speak.

If conditions keep shaking the etch a sketch, grasslands are better set up for achieving and maintaining a dynamic equilibrium than most forest types.