Look up “horse grain overload”.
This in particular seems to be commonly related to excessive grain consumption:
Also colic:
http://www.extension.umn.edu/agriculture/horse/health/preventing-and-treating-colic/
Because it was too big. In a medieval milieu there was just no way to maintain centralized control over a state that large for more than a generation. Carving it into appanages was as much practicality as it was partible inheritance and arguably even those units were awkwardly large. Since you had to rule via a certain degree of de-centralization, the unitary state could only be maintained as long as you had some semblance of unity among the ruling families. Once politics reared its ugly head, some degree of dissolution was almost inevitable. Especially as all the constituent parts of the empire had multiple other pressing issues to deal with that prevented anything like a unified drive to reassemble the state forcibly ( not that it wasn’t tried at times ).
Indeed one of the remarkable things about the Mongol Empire was actually how durable and lasting it was as an institution. The fact that there was a peaceful succession in the first generation, that a unitary state continued down to 1260 through three succesors to Genghis, that the concept and fiction of a unitary state lingered into the 14th century. That the Genghisid lineage carried real political weight right down to the modern period. Those were huge accomplishments under the circumstances. The centrifugal force of geography was immense, that the centripetal force of Genghisid dynastic unity held it together as long as it did is at least a partial testament to Genghis’ genius.
Well, re:China remember that modern China was substantially assembled by the Manchu Qing who were culturally not that dissimilar to the Mongols. And one of the pillars of the early Qing state were the Khalka Mongols in modern inner and outer Mongolia, who were absorbed early. So in a sense the incorporation of part of the Mongol state was done in alliance with “China.”
But more generally steppe pastoralists could only compete with settled societies for so long. Steppe cavalry remained at least somewhat effective militarily even into the 18th century. But the intertwining effects of a miserable resource base, sparse population and ever-lagging technological sophistication meant that rule from the steppes was ultimately doomed to failure with time. Pastoral societies were always somewhat dependant on agrarian states under even the best of circumstances - the opposite was usually not the case and became less and less so over time.
Ogedei wasn’t brilliant. Tolui was a better general, Chagatai was more devoted to tradition, Jochi was a much better corpse ;). He was just a consensus builder and was handpicked for the succession for exactly that reason. Most of his accomlishments and successes were heavily reliant on the system he inherited from his father and that included Genghis promoting him in the first place. Genghis was an unqualified genius. But genius is quite rare - his successors were often capable, but none of them were geniuses. They could not innovate and reform a system of world governance and military dominance like he could and likely never had the notion they needed to until it was too late.
It was Genghis who de-tribalized the Mongols ( or perhaps more accurately wielded them into a single super-tribe ) and created a genuinely professional army. But with time it re-tribalized as miltary commands settled down in one place and integrated into surrounding society. Genghis enforced draconian parade-ground discipline, it was Genghis who hand-picked one of history’s great officer corps. But over time these advantages eroded and as they did so did the internal strength of the state. And perhaps Genghis himself would have had increasing difficulties given another 100 years of life. It’s one thing to rule from the steppe on a conquering spree, quite another to rule from a semi-stagnant gilded throne in China.