I was going to go with Rainfall but then i realize that the soil structure is what is growing the grass so I think Soil Structure is the answer. With enough nutrients in the soil it will be able to expand (aka. grow).
I believe it is grass fires because, well... think of it... When grass catches on fire doesn't that stop forest from growing. And then once they catch on fire it can catch other things on fire like the trees in the forest. Hope this helps! =)
